Preheat the oven to 350 degrees F (175 degrees C).

Mix beef, onion, 1/2 of the tomato sauce, bread crumbs, egg, garlic, 1/2 teaspoon oregano, salt, and pepper in a large bowl. Pat into a 1/2-inch-thick rectangle on a sheet of waxed paper. Sprinkle mozzarella cheese on top.

Lift up the short side of the waxed paper to start rolling up the meatloaf. Roll tightly, peeling back paper as you go. Seal edges in place. Transfer meatloaf to a loaf pan.

Bake in the preheated oven for 1 hour. Combine remaining tomato sauce and 1/2 teaspoon oregano; pour over meatloaf and sprinkle with Parmesan cheese. Bake until cheese is melted, about 15 minutes more. Let stand for 5 minutes before serving.