Heat butter in saucepan; add barley and onion, saute until light golden color. Add nuts, dry onion soup and chicken broth; add mushrooms and water chestnuts. If using fresh mushrooms, saute in butter before adding. Stir well, turn into casserole, cover and bake, adding more liquid if necessary. If made the day before, barley has more time to absorb liquid. Bake at 350° 1 hour or longer. Leftovers freeze well.