Soak beans overnight in water. Do not drain off soaking water. Add ham bone if using, salt, onion, and vegetables. Simmer ½ hour. Mix butter and flour to smooth paste, stir into beans. Simmer until thickened, then add hot milk. If ham bone is used, remove from beans before adding milk. Take off the meat. Add bits of ham along with milk. Or top each serving with strips of ham. Serve with bread sticks or crisp crackers if desired.