Chop onion, mix with ground beef, salt, egg pepper, oregano, breadcrumbs and 1/2 cup tomato sauce. Place on waxed paper forming a rectangle. Sprinkle mozzarella cheese into the center and roll meat mixture into a loaf, sealing the ends. Place in baking pan and bake for ` hour. Pour remaining 1/2 cup tomato sauce on top and cook for 20 additional minutes. Let set for 10 minutes before serving.