In a mixing bowl, combine flour, powdered sugar, cinnamon, nutmeg, and salt until blended. Add in cold butter, rubbing it with your fingertips until the mixture resembles coarse crumbs. Stir in the egg yolk and cold water gradually until the dough comes together. Wrap the dough in plastic wrap and chill it in the refrigerator for 30 minutes to firm up.

Preheat your oven to 350°F (175°C). On a floured surface, roll out the chilled dough into a 12-inch circle. Gently fit the dough into a tart pan, trimming any excess. Line the crust with parchment paper, fill it with pie weights, and bake for 15 minutes. Remove the weights and bake for an additional 10 minutes until golden. Cool completely on a wire rack.

In a small bowl, bloom the gelatin by sprinkling it over 2 tablespoons of cold water. In a saucepan, heat the milk until just steaming, then stir in the bloomed gelatin until fully dissolved. Remove from heat and add in chopped white chocolate, stirring until melted and smooth. Mix in the vanilla extract and allow the mixture to cool to room temperature.

In a large bowl, whip the heavy cream until soft peaks form. Gently fold the cooled white chocolate mixture into the whipped cream using a spatula, being careful not to deflate the air. Continue this until no streaks remain, creating a light and airy mousse.

Pour the fluffy mousse into the cooled tart shell, smoothing the top with a spatula. Carefully cover the tart with plastic wrap and refrigerate for at least 4 hours, or until fully set.

Once the mousse is set, carefully remove the tart from the pan. Decorate with seasonal berries, chocolate curls, and mint leaves. Slice with a warm knife and serve.