Transfer lamb, fat side up, to a large roasting pan fitted with a rack, and add broth. Roast lamb 20 minutes. Reduce temperature to 325 degrees, and roast until a thermometer inserted into thickest part of flesh near the bone reaches 130 degrees to 135 degrees (medium-rare), about 55 minutes. Increase temperature to broil; broil until surface sizzles and becomes brown and crisp, about 5 minutes. Transfer lamb to a platter or carving board; let rest at least 20 minutes before carving.
Next, see our Step-by-Step Guide to Carving Lamb at the Table.