1Heat gas or charcoal grill. In tightly covered container, shake all dressing ingredients. Reserve 1/4 cup dressing. In small bowl, mix onions and tomatoes with remaining dressing; set aside.
2Place beef steak and bell pepper on grill. Cover grill; cook over medium heat 15 to 20 minutes, turning bell pepper frequently and beef once, until beef is desired doneness. Sprinkle beef with salt and pepper. Cut beef into 1/4-inch slices. Toss beef and reserved 1/4 cup dressing. Cut bell pepper into strips.
3In large bowl, place salad greens, mushrooms, bell pepper and beef. Add tomato mixture; toss.