What vegetable can goldfish eat and how should it be prepared?

Most commercial Goldfish foods/flakes contain some cereals and I have seen large goldfish eating boiled peas, boiled french beans and stale bread but I would not be trying to feed home cooked vegetables to my goldfish if wished them to live long and healthy lives. If you really wish to give your goldfish a propper treat, give them some live earthworms. (If the worms are too big for the fish to eat whole chop them up).