Preheat the grill to medium. In a small bowl, combine the olive oil, garlic, 1 teaspoon salt and 1/2 teaspoon pepper. Brush the steak and the onion slices with some of the seasoned oil. Place the potatoes and peppers on a baking sheet and toss with the remaining seasoned oil.
Arrange the peppers and onion slices on the grill, cover and cook, turning occasionally, until charred and crisp-tender, 10 minutes. Transfer to a bowl. Arrange the steak and potatoes on the grill, cover and cook for 9 minutes on each side for medium-rare.
Coarsely chop the peppers and onions; return to the bowl. Thinly slice the steak against the grain and add to the mixture. Add the basil and toss. Serve the steak, peppers and onions on top of the potatoes.