Heat the oven to 425 degrees. Remove as much of the surface fat as is practical from the lamb. Mix the spices with the salt and pepper; rub the meat all over with the spice mixture.
Put the lamb on a rack in a roasting pan. (You might line the pan first with aluminum foil to make cleanup easier.) Roast for 30 minutes, then check. If the lamb threatens to burn, turn the heat down to 350 degrees; otherwise leave it at 425 degrees.
After about 1 hour of roasting, check the internal temperature of the lamb with an instant-read thermometer. Continue to check every 10 minutes; when it reaches 130 degrees for medium-rare (125 degrees for very rare) in its thickest part (check it in several places), it’s done. The total cooking time will be less than 1 1/2 hours. Let it rest for a few minutes before carving and serving.