Heat margarine in a medium nonstick skillet over medium-high heat. Add mushrooms; sauté 5 minutes or until tender. Set aside.
Combine barley, onion soup mix, and water in a 3-quart casserole, and stir in mushroom mixture. Cover and bake at 350° for 1 hour and 15 minutes or until liquid is almost absorbed. Garnish casserole with chives, if desired.