1. In a 1 1/2 quart casserole, combine ground beef and onion. Cook 4 to 5 minutes, stirring halfway through cooking time. Spoon off drippings.
2. Stir in tomato sauce, ketchup, Parmesan cheese, garlic salt, and oregano. Cook, covered, 5 to 6 minutes, stirring halfway through cooking time.
3. In a 1-cup glass measure, combine garlic powder and butter; heat 30 seconds. Stir to blend. Pour melted butter evenly over inside of top half of each roll.
4. Divide meat mixture evenly and spread on bottom halves of rolls. Top with 1 slice Mozzarella, place tops on rolls, and wrap each in a napkin.
5. Cook as follows, rotating one-quarter turn halfway through cooking time: 30 to 45 seconds for 1 sandwich; 1 1/2 to 2 minutes for 3 sandwiches; and 3 to 4 minutes for 6 sandwiches.