Trim all excess fat from meat. Brown beef in its own fat in a large skillet; remove and set aside. Drain all but 2 tablespoons fat from skillet. Saute onion until soft in fat. Place meat, sauteed onion, potato, zucchini, red pepper rings, wine, salt, pepper and bay leaf in slow cooker; cover.
Cook on Low for 10 hours or on High for 5 hours, or until meat is tender when pierced with a fork.
Place meat and vegetables on heated platter and keep warm. Turn heat control to High. Combine flour and water in a cup. Pour into liquid in slow cooker, cover. Cook 15 minutes.
Pass sauce separately in heated gravy boat.