1 lb. lean ground beef
1 c. Quaker Oats (quick or old fashioned), uncooked
1 (15 1/2 oz.) jar spaghetti sauce
1/2 c. chopped onion
1/3 c. grated Parmesan cheese
1 egg, beaten
1 tbsp. Worcestershire sauce
1 tsp. pepper
1/3 c. of any of the following: 
Sliced mushrooms, sliced ripe olives, shredded
Mozzarella cheese, and shredded Zucchini
1/2 c. (2 oz.) shredded Mozzarella cheese
Sliced, ripe olives  

Heat oven to 350 degrees. Grease 8 inch square baking pan. Combine ground beef, oats, 1/2 cup spaghetti sauce, onion, Parmesan cheese, egg, Worcestershire, and pepper; mix well. Separate mixture into two equal parts. Shape each into a patty about 7 inches in diameter. Place stuffing choices on one patty to within 1/2 inch of edge. Top with other patty. Pinch sides together to completely enclose stuffing; smooth edges into rounded loaf. Place into prepared pan. Bake 40 to 45 minutes or until meat is done. Top loaf with 1 cup spaghetti sauce and Mozzarella cheese. Garnish with olives and serve with remaining sauce. 6 servings.
 

Source: http://www.nancys-kitchen.com