Turn the oven to fan 200C/ conventional 220C/gas 7. Heat half the oil in a flameproof roasting tin or ovenproof sauté pan. Brown the lamb for 2 minutes on each side, then lift out of the pan and set aside. Add the rest of the oil to the pan, add the potatoes, then fry for 4-5 minutes, until starting to brown. Add the rosemary and garlic, then return the lamb to the pan, too.