Trim excess fat from roast; rub beef well with flour; brown in oil in a large skillet or an electric slow cooker. Place meat in slow cooker; pour onion soup over; cover cooker. Cook on Low for 10 hours or on High for 5 hours, or until meat is tender when pierced with a two tined fork. Place green pepper rings on roast to steam during last 10 minutes of cooking.
Remove meat and pepper rings to heated serving platter; keep hot while making gravy from juices. Turn heat to high. Combine quarter cup flour and half cup cold water in a cup to make a smooth paste. Stir into slow cooker; cover; cook 15 minutes; taste and season with salt and pepper if needed.