Cook barley according to package directions. Drain and cool.
In a bowl, combine mushrooms, carrots, and green onions. Add cooked barley and toss well.
In a bowl or screw-top jar, combine salad oil, lemon juice, water, salt, dry mustard, tarragon, pepper, and garlic powder. Add to barley mixture and toss.
Cover and chill in the refrigerator for at least 3 hours or up to 24 hours. Serve on lettuce. Makes 8 to 10 side-dish servings.