HEAT barbecue to medium-high heat. Reserve 1/4 cup dressing; brush remaining dressing onto top of steak and cut-sides of peppers.
PLACE steak and peppers, dressing-sides down, on barbecue. Grill 10 min. or until steak is medium donensss (160ºF) and peppers are crisp-tender, turning steak after 5 min. (No need to turn peppers.) Meanwhile, cover 4 serving plates with greens; top with tomatoes and onions.
CUT steak across the grain into thin slices; cut peppers into strips. Arrange steak and peppers over salads. Drizzle with reserved dressing.