Thaw the puff pastry according to the package instructions. Make sure it’s still cold when you are ready to roll it out.
Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
In a small bowl whisk the flour, brown sugar, cinnamon, nutmeg, and salt together.
In a large bowl combine the apples and lemon juice. Add the flour and sugar mixture and toss to coat evenly.
On the parchment paper you prepared for the baking pan. Roll out the dough slightly about 12x12 using a rolling pin.
Using a sharp knife, cut horizontal strips about 3/4 inch wide along the pastry's left and right sides (outer thirds) of the pastry. Cut the top two and bottom two strips off and discard. Fold the top and bottom strip over the apple filling.
Spread the apple mixture down the middle one-third of the pastry lengthwise, leaving a one-inch space from the top and bottom. Fold the top and bottom strip over the apple filling.
Braid the pastry by folding over a strip from the left, then a strip from the right crossing them over each other. Continue all the way down the pastry until all the strips are over the apple mixture.
Beat the egg with 1 tablespoon of water, set aside.
Transfer the parchment back to the baking sheet and refrigerate for 20 minutes.
Remove from the refrigerator and lightly brush the top and sides of the pastry with the egg wash.
Bake for 35 to 40 minutes, until golden brown.