1. Bring potatoes and water to cover to a boil in a medium saucepan. Cook 10 to 15 minutes or just until tender. Drain, cool, and cut into 1/2-inch-thick slices.
2. Stir together olive oil and garlic. Brush half of mixture over both sides of potato slices. Sprinkle potatoes evenly with 1/4 tsp. salt and 1/4 tsp. pepper, and set aside. Brush both sides of steak with remaining half of olive oil mixture; sprinkle evenly with remaining 1/4 tsp. salt and 1/4 tsp. pepper.
3. Grill steak, covered with grill lid, over medium-high heat (350° to 400°) 6 to 7 minutes on each side or to desired degree of doneness. Cover loosely with aluminum foil, and let stand 10 minutes.
4. Grill potato slices, covered with grill lid, over medium-high heat 3 to 4 minutes on each side or until browned.
5. Cut steak diagonally across the grain into thin strips. Serve with potatoes and Red Onion Relish.