Bring potatoes and water to cover to a boil in a medium saucepan. Cook 10-15 minutes or just until tender. Drain, cool, and cut into ½-inch thick slices.
Stir together olive oil and garlic. Brush half of mixture over both sides of potato slices. Sprinkle potatoes evenly with ¼ teaspoon salt and ¼ teaspoon pepper. Set aside. Brush both sides of steak with remaining half of olive oil mixture; sprinkle evenly with remaining ¼ teaspoon salt and ¼ teaspoon pepper.
Grill steak, covered with grill lid, over medium-high heat (350ºF-400ºF) 6-7 minutes on each side or to desired degree of doneness. Cover loosely with aluminum foil, and let stand 10 minutes.
Grill potato slices, covered with grill lid, over medium-high heat 3-4 minutes on each side or until browned.
Cut steak diagonally across the grain into thin strips. Serve with potatoes and Red Onion Relish.
RED ONION RELISH:
Cut onion into ¾-1-inch thick slices. Brush both sides of onion slices evenly with olive oil.
Grill onion slices, covered with grill lid, over medium-high heat (350ºF-400ºF) 3-4 minutes on each side or until grill marks appear and onion is tender. Transfer to cutting board, and let cool slightly. Coarsely chop onion slices.
Toss together onion, feta, and remaining ingredients.