Heat gas or charcoal grill. In large bowl, place mushrooms, beef, onions and 1/2 cup of the vinaigrette; toss to coat. Let stand 10 minutes; drain. Place mixture in grill basket (grill “wok”).
Place basket on grill. Cover grill; cook over medium-high heat 7 to 9 minutes, shaking basket or stirring beef mixture twice, until vegetables are tender and beef is desired doneness. Stir in tomatoes.
Spoon beef mixture into serving dish. Stir in remaining 2 tablespoons vinaigrette. Serve with toothpicks.