Heat closed contact grill for 5 minutes.
Meanwhile, in large bowl, combine bell pepper, onion and salad dressing; toss to coat.
When grill is heated, with tongs, remove bell pepper and onion from dressing; place on bottom grill surface. Reserve remaining dressing in bowl. Close grill; cook 4 minutes.
Add mushrooms to reserved dressing; toss to coat. With tongs, add mushrooms to vegetables on grill. Close grill; cook 3 to 4 minutes or until all vegetables are tender.