1Heat gas or charcoal grill. In 2- or 2 1/2-quart saucepan, place potatoes; add enough water to cover potatoes. Heat to boiling; reduce heat to medium. Cook uncovered 5 to 8 minutes or just until potatoes are tender.
2Drain potatoes; place in medium bowl. Add 2 tablespoons of the dressing; toss to coat. Place potatoes in grill basket (grill "wok") if desired. Brush beef with 1 tablespoon of the dressing; sprinkle with salt and pepper.
3Place beef and potatoes on grill. Cover grill; cook over medium heat 8 to 15 minutes, turning once, until beef is desired doneness and potatoes are golden brown. Cut beef into thin slices.
4Among 4 plates, divide lettuce, tomatoes and onion. Top with beef and potatoes; drizzle with remaining dressing. Sprinkle with additional pepper if desired.