Set your oven to 400 degrees
Place a sheet of parchment paper on a baking sheet and top that with one of the pastry sheets
Cut about 1/2 inch slices out of the second puff pastry and form a border around the pastry sheet on the baking pan - Kind of like a picture frame
Make an egg wash with the egg and water by mixing it
Spread the egg wash over the pastry
Poke lots of holes in the pastry with a fork
On another baking sheet, place your squash and onion slices
Drizzle with olive oil, and sprinkle salt and pepper for seasoning
Bake both the pastry and the vegetables for about 15 minutes
Pull the pastry out - If it has puffed up, poke some more air in the pastry and push down to release air pockets
Bake the squash for an additional 10 minutes
Spread the cheese on the inside of your pastry border
Top with squash, and onions
Cut into squares and serve