Place the racks of lamb in a baking tray/dish, scatter garlic and rosemary and drizzle with olive oil. Allow to marinate for at least 4 hours, turning occasionally. Preheat the oven to fan 200C/conventional 220C/gas 7. Generously season the lamb with salt and pepper. Brown the rack in a hot frying pan, about 1-2 minutes on the meaty sides, turning to brown all sides. Put the rack in a roasting tin, standing in pairs with bones interlinked. Roast for 15 minutes for very pink meat, 20 minutes for medium-rare, 25 minutes for medium/well-done and 30 minutes for well done. Let the lamb rest for 10 minutes before carving into cutlets.