In a large mixing bowl, combine flour, powdered sugar, cinnamon, nutmeg, and salt. Whisk to blend.

Add cold butter cubes. Use a pastry cutter or your fingertips to rub the butter into the flour mixture until it resembles coarse crumbs.

Add the egg yolk and 1 tablespoon cold water. Mix gently with a fork.

Add more water only if needed for the dough to hold together when pinched.

Form the dough into a disk, wrap in plastic, and chill in the refrigerator for at least 30 minutes.

Preheat oven to 350°F (175°C).

Roll out the chilled dough on a lightly floured surface to fit a 9-inch tart pan with a removable bottom.

Press the dough gently into the tart pan. Trim excess dough from the edges. Prick the base with a fork and line it with parchment paper.

Fill with pie weights or dried beans. Bake for 15 minutes.

Remove parchment and weights, and bake for an additional 10 minutes until golden brown.

Let the crust cool completely on a wire rack.

If using gelatin, sprinkle it over 1 tablespoon of cold water in a small bowl. Let it bloom for 5 minutes.

In a small saucepan, heat the milk until just simmering (do not boil).

Remove from heat and stir in the bloomed gelatin until dissolved.

Add chopped white chocolate to the warm milk. Let sit for a minute, then stir until smooth and melted.

Stir in vanilla extract and allow to cool to room temperature.

In a large bowl, beat chilled heavy cream to soft peaks using a hand mixer.

Gently fold the whipped cream into the cooled white chocolate mixture in batches, being careful not to deflate the mousse.

Spoon or pour the mousse into the cooled tart shell.

Smooth the top with an offset spatula or back of a spoon.

Refrigerate the tart for at least 4 hours, or until the mousse is fully set and firm to the touch.

Before serving, decorate the tart with fresh berries, chocolate curls, and mint leaves if desired.

Slice with a warm knife for clean cuts. Serve chilled.