In a mini food processor, combine garlic, rosemary and olive oil and process until garlic is finely chopped. Season lamb racks with salt and pepper and rub garlic-rosemary oil all over them. Set racks fat-side up on a large rimmed baking sheet and let stand for 1 hour.
Preheat oven to 450°F. Roast lamb in upper third of oven for 15 minutes. Turn racks and roast for 10 minutes longer for medium-rare meat. Transfer racks to a carving board, stand them upright and let rest for 10 minutes.
Carve racks in between rib bones and transfer to plates. Serve immediately.