Barley with Mushrooms (Scotland)

8 ounces mushrooms, sliced
2 medium onions, chopped
2 tablespoons butter or margarine
3 1/4 cups boiling water
1 cup barley
2 teaspoons instant chicken bouillon
1/2 teaspoon salt
Dash of pepper

Cook and stir mushrooms and onions in butter 5 minutes; place in ungreased 1 1/2-quart casserole. Stir in remaining ingredients. Cover and bake at 375 degrees F, stirring once, until barley is tender, about 1 1/4 hours.