Preheat the oven to 400 degrees. Roll out the dough to 1/8 inch thickness. Cut 16, 4 1/2 inch circles, rerolling the dough as necessary. Place 1/2 a slice of cheese on 8 rounds, top with 1/4 cup of the onion filling, and place another 1/2 slice of cheese on top. Brush egg wash around the edge of the dough. Cover with another dough round and press the dough edges together to seal. Using a fork, crimp the edges. Brush the tops with egg wash, place on a lined baking sheet, and bake for 15-20 minutes or until browned. Remove from oven and cool slightly before serving.