In a 1½- or 2-quart slow cooker, combine mushrooms, onion and sweet pepper. Top with meatballs. In a small bowl, combine tomato sauce, tomato paste, Italian seasoning and garlic. Pour over mixture in cooker.
Cover and cook on low-heat setting for 4 to 5 hours or on high-heat setting for 2 to 2½ hours. If no heat setting is available, cook for 3 to 3½ hours.
To serve, cook spaghetti according to package directions; drain. Divide spaghetti between serving plates. Top with meatball mixture. If desired, garnish with basil.