The first time I made this combo I thought, "Why haven’t I been doing this forever?"
Sweet, deeply browned onions meet meaty baby bellas and the whole kitchen smelled like comfort.
This simple skillet recipe turns ordinary weeknight sides into something that feels deliberate and luxurious.
It’s quick to pull together, scales easily, and finishes with a hit of balsamic that brightens the whole pan.
Use it as a burger topper, a filling for warm tarts, or spooned over roasted chicken for instant flavor depth.
If you want ideas for serving, my favorite is pairing it with a steak crostini with creamy horseradish sauce and caramelized onions for a party appetizer that disappears fast.
Why You’ll Love This Recipe
- Deeply caramelized onions bring sweet, sticky layers of flavor.
- Baby bella mushrooms add a meaty, savory chew that soaks up butter and balsamic.
- A small amount of fat and low, patient heat create real caramelization without burning.
- Versatile: use it as a side, sandwich topper, pizza scatter, or tart filling.
- Fast-ish: active hands-on time is short; most cooking is slow and forgiving.
The finished texture is a mix of tender, jammy onion ribbons and thick, slightly chewy mushroom slices.
On the palate you’ll get sweet, umami, and a tangy lift from the balsamic that keeps things from feeling cloying.
It’s rich without being oily because the mushrooms surrender moisture and concentrate flavor as they brown.
"Five stars — this took plain dinner to restaurant-level in 20 minutes. My whole family loved it!" — reader Sarah C.
Key Ingredients
Yellow onion
A large yellow onion brings the right ratio of sugar and bulk to caramelize evenly. Choose onions that feel heavy for their size and have dry, papery skins; they give the sweetest, most complex flavor when cooked low and slow.
Baby bella (cremini) mushrooms
Baby bellas are my go-to for this recipe because they have a drier, earthier profile than white buttons and hold up to sauteing without turning into mush. If you can, buy loose mushrooms so you can check for firmness and minimal spotting.
Butter + olive oil
A combination of salted butter and olive oil gives you the nutty, toasty flavor of butter plus the higher smoke point and coating power of oil. Use a good European-style butter for a richer mouthfeel, and a fruity extra-virgin olive oil to layer flavor.
Balsamic vinegar
Just a teaspoon at the end brightens the pan and adds a glossy finish. Choose a decent-quality, aged balsamic for deeper notes; you don’t need an expensive bottle, but avoid the watery supermarket jug.
Full ingredient list:
- 2 tablespoons salted butter
- 2 tablespoons olive oil
- 4–5 ounces baby bella (cremini) mushrooms
- 1 large yellow onion (or 2 small onions)
- 1/2 teaspoon salt
- 1/8 teaspoon black pepper
- 1/8 teaspoon dried thyme
- 2 garlic cloves
- 1 teaspoon balsamic vinegar
Step-by-Step Instructions
Step 1: Prep the vegetables
Trim the bottom 1/8 inch of each mushroom stem and discard that tough stub.
Slice each mushroom into roughly 1/4-inch slices so they cook evenly and brown consistently.
Peel the onion, halve it from root to tip, then slice each half into 1/4-inch thin rings.
Pro Tip: If your mushroom slices vary widely in thickness, push the thicker pieces to one side of the skillet; they’ll take a minute or two longer to brown.
Step 2: Start the onions low and slow
Warm a large stainless steel skillet over medium-low heat.
Add the butter and 1 tablespoon of olive oil, allowing the butter to melt until foaming is nearly gone.
Add the sliced onions and sauté undisturbed for 10 minutes, stirring halfway through to prevent any hot spots.
Pro Tip: You’ll know they’re ready for the next stage when the onions are translucent and starting to pick up golden edges, not dark brown streaks.
Step 3: Season and continue caramelizing
Sprinkle the onions with 1/2 teaspoon salt, 1/8 teaspoon black pepper, and 1/8 teaspoon dried thyme.
Continue cooking on medium-low for another 10 minutes, stirring every 2–3 minutes to promote even browning.
Once the onions are soft, jammy, and a rich amber, transfer them to a plate and set aside.
Pro Tip: If the onions begin to crisp too quickly or smell burned, lower the heat and add a teaspoon of water to deglaze and regulate temperature.
Step 4: Brown the mushrooms
Add the remaining 1 tablespoon olive oil to the same skillet and spread it so the pan is lightly coated.
Arrange the mushroom slices in a single layer; avoid crowding or they’ll steam instead of brown.
Cook for about 4 minutes without moving them so they form a brown crust, then flip and cook another 3 minutes.
Pro Tip: You’ll know it’s time to flip when the exposed edges of the mushrooms are deeply browned and release easily from the pan.
Step 5: Finish everything together
Return the caramelized onions to the skillet with the mushrooms.
Toss in the minced garlic and drizzle 1 teaspoon balsamic vinegar over the mixture.
Cook together for another 3–5 minutes, stirring frequently to combine flavors and reduce any excess moisture.
Pro Tip: Taste and adjust seasoning at the end — a pinch more salt or a squeeze of lemon brightens the whole dish if needed.
Step 6: Serve
Spoon the mixture over steaks, burgers, roasted vegetables, polenta, or inside warm tarts.
This also freezes well in small portions for quick weeknight finishes.
Pro Tip: For a crisp contrast, try sprinkling toasted breadcrumbs or crisp onions on top right before serving.
Expert Tips for Success
- Use a heavy-bottomed skillet: A stainless steel or cast-iron pan distributes heat evenly and helps build those brown fond bits for flavor. Thin pans create hot spots and increase the chance of burning.
- Control the heat: Caramelization is a slow reaction. Keep the stove at medium-low for the onions and don’t rush them with higher heat; patience yields sweeter, more complex flavor.
- Dry your mushrooms: Pat mushrooms completely dry with paper towels before slicing. Moisture prevents browning and leads to steaming.
- Don’t overcrowd the pan: When mushrooms are layered, they steam. Cook in batches if necessary so each slice gets contact with the hot pan.
- Salt at the right time: Add a little salt early to pull moisture from the onions, then adjust more at the end. For mushrooms, a little salt at the start can help drawing flavor but avoid salting too heavily before browning.
- Garlic and balsamic at the end: Add garlic and vinegar in the last few minutes so garlic doesn’t burn and balsamic retains brightness instead of reducing to an overpowering syrup.
- Equipment note: A long-handled stainless spatula helps scrape up fond without tearing the mushrooms. If you’re curious about crispy onion techniques for toppings, check this air-fryer crispy onions recipe.
- Make larger batches and freeze: This recipe is forgiving and freezes well in small airtight containers to use as quick flavor boosters on busy nights. I often pair leftovers with a tart or warm grain bowl — the same caramelization principle works with sweet-savory bakes like baked apples with feta, honey and cranberries when I want a contrast.
Storage & Freezing
Fridge storage: Transfer cooled caramelized onions and mushrooms to an airtight container and refrigerate for up to 4 days.
Label with the date so you use them while they’re freshest.
Freezer storage: Portion into small, freezer-safe containers or heavy-duty freezer bags.
Remove as much air as possible and freeze for up to 3 months. Thaw in the refrigerator overnight.
Reheating: Reheat gently in a skillet over medium-low heat, adding a teaspoon of water or broth if the mixture seems dry.
Avoid microwaving for long periods; a skillet refreshes texture and revives the browned bits.
Variations & Substitutions
- Vegan swap: Replace butter with an equal amount of neutral-flavored oil and add a tablespoon of vegan butter at the end for richness. Dried thyme and a splash of soy sauce can deepen umami.
- Add fresh herbs: Stir in a tablespoon of chopped fresh parsley, tarragon, or chives at the end to add herbal brightness.
- Swap mushrooms: Shiitake or portobello work well for more pronounced umami. Slice thicker portobellos and brown a little longer.
- Make it smoky: Add a teaspoon of smoked paprika or finish with a few drops of liquid smoke for a barbecue-like depth.
Frequently Asked Questions
Q: How long should onions take to fully caramelize?
A: With medium-low heat, expect about 20–25 minutes for true caramelization. The process is slow because you’re encouraging the onion’s natural sugars to break down and brown without burning.
Q: Can I use white button mushrooms instead of baby bellas?
A: Yes, but button mushrooms are milder and higher in water content. Dry them thoroughly and give them a few extra minutes in the pan to develop color.
Q: Why did my mushrooms get soggy?
A: Sogginess usually means the pan was too crowded or the mushrooms weren’t dry. Cook in batches and ensure the pan is hot enough so slices brown instead of release moisture and steam.
Q: Can I add wine or stock while cooking?
A: A splash of dry white wine or mushroom stock works as a deglazer after the mushrooms brown. Add sparingly so you don’t lose the concentrated flavor — cook it down until the liquid is almost gone.
Q: Can I make this ahead for a party?
A: Absolutely. Make it a day ahead and reheat gently on the stovetop. It actually tastes more cohesive the next day as flavors meld.
Final Thoughts
This caramelized onions and mushrooms recipe is one of those small things that upgrades a meal without adding fuss.
Make a batch, and you’ll find yourself reaching for it again and again.
Please leave a star rating in the recipe card below if you try it, and pin this to Pinterest to save it for later!
Conclusion
For another excellent perspective on technique and flavor balance, see Classic Caramelized Onions and Mushrooms – Forager | Chef.
If you want an alternate recipe and plating ideas, check out Caramelized Onions and Mushrooms – The Culinary Compass.
Caramelized Onions and Mushrooms
Ingredients
For the caramelized mixture
- 2 tablespoons salted butter Use a good European-style butter for richer flavor.
- 2 tablespoons olive oil Use a fruity extra virgin olive oil.
- 4–5 ounces baby bella (cremini) mushrooms Choose firm, unblemished mushrooms.
- 1 large yellow onion Or use 2 small onions.
- 1/2 teaspoon salt Adjust to taste.
- 1/8 teaspoon black pepper
- 1/8 teaspoon dried thyme
- 2 cloves garlic Minced.
- 1 teaspoon balsamic vinegar Use quality aged balsamic.
Instructions
Preparation
- Trim the bottom 1/8 inch of each mushroom stem and discard. Slice each mushroom into roughly 1/4-inch slices.
- Peel the onion, halve it from root to tip, then slice each half into 1/4-inch thin rings.
Cooking the Onions
- Warm a large stainless steel skillet over medium-low heat and add the butter and 1 tablespoon of olive oil.
- Allow the butter to melt until foaming is nearly gone, then add the sliced onions.
- Sauté undisturbed for 10 minutes, stirring halfway through.
Seasoning and Caramelizing
- Sprinkle the onions with salt, black pepper, and dried thyme. Cook for another 10 minutes, stirring every 2-3 minutes.
- When onions are soft and a rich amber, transfer them to a plate.
Brown the Mushrooms
- Add the remaining tablespoon of olive oil to the skillet and spread it around.
- Arrange the mushroom slices in a single layer and cook for about 4 minutes without moving them.
- Flip the mushrooms and cook for another 3 minutes.
Combine the Mixture
- Return the caramelized onions to the skillet and add the minced garlic and balsamic vinegar.
- Cook together for another 3-5 minutes, stirring frequently.
Serve
- Spoon the mixture over steaks, burgers, or inside warm tarts.
- This also freezes well in small portions.
