Steam asparagus, covered, for 2 minutes. Drain and plunge into ice water; drain well. Combine the asparagus, mushrooms, and the next 5 ingredients (mushrooms through pepperoncini peppers) in a large bowl.
Combine vinegar and remaining ingredients in a small bowl; stir well with a whisk. Pour vinaigrette over the vegetable mixture, tossing gently to coat. Cover and marinate in refrigerator 2 hours; stir occasionally.
Note: Can be served chilled or at room temperature.
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