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Heat oven to 220C/fan 200C/gas 7. Heat half the oil in a roasting tray, then brown the steaks on both sides and set aside. Toss the potato, onion, remaining oil and half the rosemary into the roasting tray, then lay the steaks on top. Brush the steaks with the mustard, scatter with remaining rosemary and roast everything for 20 mins until the potatoes are softened and the lamb cooked.