1. PREHEAT grill to medium-high heat. Reserve 1/3 cup of the dressing. Brush remaining dressing lightly onto one side of the steak and onto cut sides of peppers.
2. PLACE steak and peppers on grill, dressing-sides down. Grill steak 5 min. on each side or until medium doneness (160°F) and grill peppers 10 min. (Peppers do not need turning.) Meanwhile, cover four serving plates with greens; top with tomatoes and onions.
3. CUT steak across the grain into thin slices; cut peppers into strips. Arrange steak and peppers over salads. Drizzle with the reserved 1/3 cup dressing.