Preheat your oven to 350°F (175°C). In a large mixing bowl, whisk together flour, powdered sugar, cinnamon, nutmeg, and salt until well combined. Cut in cold butter until mixture resembles coarse crumbs. Add egg yolk and cold water gradually, mixing until a dough forms. Wrap in plastic wrap and chill for 30 minutes.

Roll out the chilled dough on a lightly floured surface until about ¼ inch thick. Fit the rolled dough into a tart pan, pressing it firmly against the edges. Place parchment paper over the crust and fill with weights. Bake for 15 minutes, remove weights and parchment, then bake for an additional 10 minutes until golden brown. Cool completely on a wire rack.

If using gelatin, bloom it by sprinkling it over a small amount of cold water in a bowl. In a saucepan, heat milk over medium heat until it simmers. Remove from heat and stir in bloomed gelatin until dissolved. Add chopped white chocolate and mix until fully melted. Let cool to room temperature while whipping the cream to soft peaks.

Once the chocolate mixture has cooled, gently fold in the whipped cream using a spatula until no white streaks remain.

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

Once chilled, remove the tart from the pan, garnish with fresh berries and chocolate curls. Serve chilled.