In a heavy skillet, saute 1/2 cup chopped onion and 1 clove garlic (minced) in two tablespoons butter. Add 1 pound ground beef; cook until brown. Then stir in one 6 ounce can tomato paste, 3 1/2 cups canned tomatoes, 1/2 cup water, 1 teaspoon salt, 1/4 teaspoon pepper, and 1/2 teaspoon oregano. Simmer the sauce about 1 hour, stirring occasionally. Serve sauce over hot spaghetti. Top with grated Parmesan cheese. Makes sauce for 6 servings.