In a skillet, saute mushrooms and onions in butter until tender. Stir in barley and pimientos.
Transfer to a greased 13-in. x 9-in. baking dish. Dissolve 3 teaspoons bouillon in 2 cups water; stir in barley mixture. Cover and bake at 325° for 1 hour. Dissolve remaining bouillon in remaining water; stir into barley mixture.
Bake, uncovered, 30 minutes longer or until liquid is absorbed and barley is tender.