Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Line a sheet pan with parchment paper.

In a small bowl, beat the egg and water together with a fork.

In a medium bowl, combine the brown sugar, flour, and cinnamon. Add the apple slices and the walnuts, and toss to coat.

Dust your work surface with flour and unfold the puff pastry. Lightly dust the top of the dough with flour and roll out into a 16-by-12-­inch rectangle. Place the dough on the parchment-lined sheet pan.

Arrange the coated apples lengthwise, down the center of the dough.

Cut slits 1 inch apart along the two sides of the rectangle to within ½ ­inch of the apples.

Starting at one end, fold the strips over the apples, alternating sides, to “braid” the dough. Continue to fold the dough strips until the apples are covered completely. Tuck the end strips under and brush the pastry with the egg wash. Sprinkle with coarse sugar.

Bake for 35 minutes, or until golden brown, rotating halfway.

Remove from the oven. Place the sheet pan on a wire rack and allow to cool for 20 minutes before slicing and enjoying.