Prepare the Puff Pastry: Preheat your oven to 400°F (200°C). Roll out the thawed puff pastry sheets on a lightly floured surface. Using a sharp knife, cut out Christmas tree shapes (about 4-6 inches tall) from the pastry. Transfer them to a baking sheet lined with parchment paper.
Prepare the Apples: In a medium bowl, toss the thinly sliced apples with 1 teaspoon of lemon juice to prevent browning.
Assemble the Tartlets: Place a layer of brie cubes at the base of each puff pastry tree. Then, top with a generous layer of apple slices. Drizzle 1 tablespoon of caramel sauce over each tree, ensuring an even coating.
Finish with Pastry: Bake the tartlets in the preheated oven for 15-20 minutes, or until the pastry is golden brown and puffed. Remove from the oven and let cool for a few minutes.
Add the Finishing Touches: Brush the edges of the pastry with a beaten egg for a shiny finish and sprinkle brown sugar on top. Drizzle with additional caramel sauce and garnish with fresh rosemary sprigs.