PREHEAT barbecue to medium-high heat. Reserve 1/4 cup of the dressing. Brush remaining dressing lightly onto one side of the steak and onto cut sides of peppers.
PLACE steak and peppers on barbecue, 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.
CUT steak across the grain into thin slices; cut peppers into strips. Arrange steak and peppers over salads. Drizzle with the reserved 1/4 cup dressing.