Peel and slice apples thin. You want about 15 slices per apple. Toss together with lemon juice, brown sugar, flour and cinnamon. Allow to sit for at least 15 minutes to macerate in the sugar and spices.
Preheat the oven to 400 degrees. Unroll sheet of puff pastry onto a cookie sheet lined with parchment.Arrange apple slices on the puff pastry, leaving an inch all the way around. Fold up the edges. Brush the beaten egg around the edges of your folded up puff pastry. You won't use all of the egg.
Bake for about 30 minutes, rotating the pan 180 degrees after 15 minutes. Enjoy, possibly with some ice cream or caramel sauce
Notes
*I hadn't even thawed my puff pastry the first time I made this, I just put it near my oven as it was preheating and it thawed out in about 15 minutes.*If you don't have whole wheat flour, all-purpose flour works just fine.