How to Make It
In a resealable plastic bag, combine the lamb with the garlic, rosemary, vinegar, 1/4 cup of the oil and very generous pinches of salt and pepper. Seal the bag and let sit at room temperature for 1 hour. Drain the lamb, reserving the marinade.
Preheat the oven to 425°. Heat the remaining oil in a large ovenproof skillet until shimmering. Add the lamb and cook until browned all over, about 6 minutes, turning once. Add the marinade, transfer to the oven and roast for about 5 minutes longer for medium-cooked lamb.