For vinaigrette, whisk together vinegar, honey and mustard in a small bowl. Gradually whisk in 3 tablespoons oil in a slow steady stream. Add salt and pepper to taste. Set aside.
Brush onion with additional oil. Place on a small baking sheet. Roast at 350° for 30 minutes or until tender. Allow to cool slightly. Chop coarsely; set aside.
Meanwhile, bring broth to a boil in a Dutch oven. Add barley. Reduce heat; cover and simmer 35-40 minuets or until tender. Drain any excess liquid. Spread barley on a baking sheet and allow to cool.
In a skillet, saute mushrooms in remaining 1 tablespoon oil until tender. In a bowl, combine mushrooms, onion, barley, parsley, salt and reserved vinaigrette. Toss well to mix. Serve at room temperature.
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