Preheat oven to 400°F (200°C). Line a baking sheet with parchment.
Melt butter in a skillet over medium, add brown sugar and lemon; cook until caramel forms, about 2 minutes.
Toss apple slices in caramel and a pinch of salt; cook 2 minutes more until slightly softened.
Cut puff pastry into 12 squares. Place a brie wedge on each square, top with 2–3 apple slices and a little caramel from the pan.
Fold corners toward center or leave open-faced; brush with egg wash.
Bake 12–15 minutes until pastry is golden and brie is melty. Cool 2 minutes before serving.