Cook the onion and mushrooms in the butter until onion is wilted and mushroom juices have evaporated. Add the barley and brown lightly. Pour the mixture into a buttered casserole. Pour onecup of the stock over the barley and cover. Bring to a boil.
Bake the casserole, covered, for twenty-five to thirty minutes. Add remaining stock. Continue to cook, covered, for fifteen minutes or longer, until liquid is absorbed and barley is tender.