Heat gas or charcoal grill. In 2-quart saucepan, place potatoes and enough water to cover. Heat to boiling. Reduce heat to medium; cook 5 to 8 minutes or just until potatoes are fork-tender.
Drain potatoes; place in medium bowl. Gently stir in 2 tablespoons of the dressing to coat. Brush steak with 1 tablespoon of the remaining dressing; sprinkle with salt and pepper.
Place steak and potatoes on grill over medium heat. Cover grill; cook 8 to 15 minutes, turning once, until steak is desired doneness and potatoes are golden brown.
Arrange lettuce, tomatoes and onion on large serving platter. Cut steak into thin slices; arrange on platter. Top with potatoes. Drizzle salad with remaining dressing. If desired, sprinkle with additional black pepper.