Roast in the oven for 15 minutes; lower the oven temperature to 350 degrees and roast for 1 hour more, basting from time to time with the juices. (Cook slightly longer, depending on how well done you like your meat.) Let rest in a warm place for at least 15 minutes before carving.
04Meanwhile, taste the juices, adding salt as needed. (It should not need much because of the anchovies.) If the gravy is too thin, simmer it in a skillet to thicken. If you choose, garnish the slices of lamb with sprigs of watercress. Serve with gravy and mashed potatoes or creamed eggplant.