Line 2 sheet pans with parchment paper. Roll the dough into a 10 1/2 by 14 1/2-inch rectangle between 1/8 and 1/4 inch thick and transfer it to a prepared sheet pan. Freeze the dough for 10 minutes. Cut the dough in half crosswise and trim the pieces to make two 10 by 7-inch rectangles. Transfer one of the rectangles to the other prepared sheet pan. Preheat the oven to 425°F.
Spread the filling over one of the rectangles, leaving about a 1-inch border all around. Refrigerate.
Put the second rectangle of dough in the freezer for about 5 minutes, or just long enough to firm it slightly, but not so long that it cracks when you fold it. Fold it in half lengthwise to create a 5 by 7-inch strip of folded dough. With a long, sharp knife, cut slits about 3/4 inch apart, cutting through the top and bottom layers of the dough but not cutting all the way to the fold. Leave about 3/4 inch of the fold and of the open side uncut to create the eventual center strip of dough and the border. Chill the dough so it's firm when you transfer it.
Brush egg wash over the edges of the bottom dough with the filling. Place the folded dough over the filling and wait a minute or two if it's too stiff to work with. Unfold it so there's a strip of dough running up the center with the slats on both sides. Gently press the edges to seal the top to the bottom and cut away a tiny strip along all 4 edges, so the top and bottom edges are flush. Brush the top with egg wash. Freeze for 15 minutes.
Bake for about 30 minutes, or until puffed and golden brown. Turn down the oven to 300°F and bake for 20 minutes more. Cut into rectangles to serve.