Heat the oven to 400 degrees F. Beat the egg and water in a small bowl with a fork or whisk.
Stir the muenster cheese, parmesan cheese, onion and garlic powder in a medium bowl.
Unfold the pastry sheet on a lightly floured surface. Brush the pastry sheet with the egg mixture. Top with the cheese mixture and spinach. Starting with a short side, roll up like a jelly roll. Cut into 20 (1/2-inch) slices. Place the slices, cut-side down, onto baking sheets. Brush the slices with the egg mixture.
Bake for 15 minutes or until the pastries are golden brown. Remove the pastries from the baking sheets and let cool on wire racks for 10 minutes.
For a flavor twist, try a different cheese. Any shredded cheese will work in this recipe.