Combine beef, potatoes, onion, salt, and pepper in a Dutch oven. Cook until beef is browned, stirring to crumble; drain well.
Roll pastry to 1/8-inch thickness on a lightly floured surface. Cut twenty 4 1/2-inch circles. Spoon 1 tablespoon meat mixture on each pastry circle. Sprinkle each pastry circle evenly with about 1 teaspoon water. Moisten edges of circles; fold pastry in even halves. Using a fork dipped in flour, press edges together to seal.
Place on lightly greased baking sheets. Bake at 375° for 1 hour or until lightly browned. Serve with catsup, if desired.