In a large mixing bowl, combine flour, powdered sugar, cinnamon, nutmeg, and salt until well mixed. Cut in cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Add the egg yolk, then gradually stir in cold water until the dough comes together. Shape it into a disc, wrap it in plastic, and chill in the refrigerator for 30 minutes.

Preheat your oven to 350°F (175°C). Roll out the chilled dough on a lightly floured surface to fit your tart pan. Carefully press the dough into the pan and dock the base with a fork to prevent puffing. Line with parchment paper, fill with pie weights, and bake for 25 minutes until lightly golden. Allow the tart shell to cool completely on a wire rack.

If using gelatin, bloom it by soaking in cold water as per package instructions. In a saucepan, heat the milk over medium heat until just simmering. Add the bloomed gelatin (if used) and stir until dissolved. Remove from heat, then add chopped white chocolate, stirring until melted and smooth. Let the mixture cool slightly before proceeding to fold in whipped cream.

Once the white chocolate mixture has cooled to room temperature, gently fold in the whipped cream until fully combined and fluffy. Pour this delightful mousse into the cooled tart shell, smoothing the top with a spatula for an elegant finish. Cover with plastic wrap and refrigerate for at least 4 hours or until set.

After the mousse has set beautifully, remove the tart from the refrigerator. Decorate the top with fresh berries, chocolate curls, and mint leaves for a vibrant presentation. For a clean slice, use a warm knife to cut through the tart and serve this luxurious Elegant White Chocolate Mousse Tart to your guests.