For Vinaigrette: Whisk together vinegar, honey and mustard. Slowly add oil while whisking, until emulsified. Add salt and pepper to taste.
Preheat oven to 350°F. Peel onion half and coat with oil. Place on baking sheet and roast until tender, about 30 minutes. Chop coarsely.
In a medium pot, bring broth to a boil and add barley. Reduce heat and simmer, covered, until barley is tender (35 to 40 minutes). Drain any excess liquid; spread barley on a baking sheet to cool.
In a sauté pan over medium heat, cook mushrooms with 1 tablespoon of oil until tender. Mix sautéed mushrooms into cooled barley, along with onion, parsley, salt and Vinaigrette. Adjust seasoning as desired.