Combine flour, powdered sugar, cinnamon, nutmeg, and salt in a mixing bowl. Cut in cold butter until the mixture resembles coarse crumbs. Add egg yolk and a splash of cold water, mixing until a dough forms. Shape into a disk, wrap in plastic wrap, and chill for 30 minutes.

Preheat oven to 350°F (175°C). Roll out chilled dough on a floured surface to fit the tart pan. Press into the pan neatly and cover with parchment paper and pie weights. Bake for 25 minutes until golden. Let cool completely.

If using gelatin, soak it in cold water for 5 minutes. Heat milk in a saucepan until simmering, then stir in dissolved gelatin. Melt white chocolate and mix with milk. Cool to room temperature.

In a chilled bowl, whip heavy cream until soft peaks form. Gently fold in white chocolate mixture until combined without deflating.

Pour mousse into the cooled tart shell, smoothing the top. Cover with plastic wrap and refrigerate for at least 4 hours until set.

Remove tart from fridge, garnish with fresh berries or chocolate curls. Slice with a warm knife for clean edges and serve.