Heat oven to 400°F (200°C). Line a baking sheet with parchment.
Mix almond flour, granulated sugar, butter, egg yolk, vanilla, and pinch of salt until smooth.
Cut puff pastry into four squares. Dock centers with a fork leaving a small border.
Spread a thin layer of frangipane on each square, keeping the border clear.
Toss apple slices with lemon juice, then arrange on top in overlapping rows.
Brush borders with beaten egg. Sprinkle brown sugar over apples.
Bake 18–22 minutes until pastry is puffed and golden. Cool slightly, dust with powdered sugar.