Heat closed medium-size contact grill for 5 minutes. In medium bowl, toss onion with dressing. Remove onion from dressing with tongs; place on grill. Reserve dressing in bowl. Close grill. Grill 4 minutes.
Meanwhile, toss mushrooms with dressing in bowl. Add mushrooms to grill with onions. Close grill. Grill 2 to 4 minutes or until vegetables are tender. Remove vegetables from grill; cover to keep warm.
Brush beef steaks with remaining dressing in bowl. Sprinkle both sides of steaks with salt, chili powder, garlic pepper and cumin. Place on grill. Close grill. Grill 3 to 5 minutes or until desired doneness. Serve onions and mushrooms with steaks.