Roll out the Puff Pastry (1 package) on a piece of parchment paper. Cover with aluminum foil and store it in the refrigerator.
Zest the Lemon (1) and squeeze the juice into a small dish. Set zest and juice aside.
Peel the Red Apple then remove the core.
Preheat the oven to 400 degrees F (200 degrees C).
Slice the Red Apple (4) and sprinkle lemon juice on them to keep them from browning.
Mix together the Lemon zest, the Brown Sugar (1/2 cup) and the Ground Cinnamon (1/2 teaspoon). Toss with the apple slices.
Take the puff pastry out of the fridge and pile the apple slices on the middle of the dough.
Whisk the Egg (1) with a bit of water. Fold over the edges of the dough and brush with the egg wash.
Sprinkle Granulated Sugar (to taste) on top. Cover with aluminum foil and bake for 30 minutes.
Turn down the heat to 350 degrees F (180 degrees C) and bake for 20 more minutes.
Remove foil for the last 10 minutes.
Slice and serve hot. Top with ice cream if you want to.