Heat the oven to 400°F. Beat the egg and water in a small bowl with a fork.
Stir the Muenster cheese, Parmesan cheese, onion and garlic powder in a medium bowl.
Sprinkle the flour on the work surface. Unfold the pastry sheet on the work surface. Brush the pastry sheet with the egg mixture. Top with the cheese mixture and spinach. Starting at a short side, roll up like a jelly roll. Cut into about 20 (1/2-inch) slices. Place the slices, cut-side down, onto 2 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.