Trim fat from roast. Cut roast into 1-inch cubes. In a 3-1/2- to 4-quart electric crockery cooker layer beef, onions, and red sweet pepper. Add spaghetti sauce. Cover and cook on low-heat setting for 9 to 10 hours or on cook high-heat setting for 5 hours.
If cooking on low, turn cooker to high. Add zucchini; cook 1 hour more or until meat is tender. Season to taste with salt and pepper. Serve over hot cooked spaghetti, if desired. Makes 6 to 8 servings.