Melt half the butter in a heavy frying pan.
Pour in the barley and cook slowly until it turns a golden brown, turning it frequently; spoon into large buttered casserole.
Put 2 tablespoons more of the butter into the frying pan; add chopped onions and saute until clear; add to the barley.
Melt the remaining 2 tablespoons butter; add drained mushrooms (reserving liquid) and saute for 5 minutes; add to barley and onion.
Add enough water to reserved mushroom liquor to make 1 cup liquid and combine with the chicken stock.
Pour 2 cups of the liquid over the barley.
Cover tightly and bake 45 minutes in a moderate oven (350°); add 2 more cups of liquid and stir; bake, covered, another 45 minutes.
Stir in remaining cup of liquid and bake, covered, 30 minutes longer.
You might add 1/2 cup of toasted slivered almonds when you stir in the last cup of liquid.