Combine beef, onion, mozzarella, bread crumbs, garlic, parsley and egg. Stuff partially cooked pasta shells with spoonful of meat mixture. Pour one jar of spaghetti sauce in greased 4.5 to 6 quart slow cooker. Place stuffed shells on top of sauce. Pour remaining sauce evenly over pasta. Sprinkle with Parmesan cheese. Cover and cook on low heat 4-5 hours. Do not overcook.
Serve with Caesar salad and garlic bread.