Score meaty side of lamb in a diamond pattern of 1/4-inch-deep cuts about 1 1/2 inches apart.
Season with salt and pepper, and place scored-side up in a large roasting pan.
In a small bowl, whisk olive oil with lemon juice, lemon zest, garlic, harissa, coriander seeds and cumin.
Pour marinade over lamb and massage into crevices.
Cover pan with aluminum foil and refrigerate 2 1/2 hours or overnight.
Remove from refrigerator 1 1/2 hours before cooking to return lamb to room temperature; in last 15 minutes, heat oven to 450 degrees.
Remove foil from pan and place pan on middle oven rack; turn heat down to 350 degrees.
Roast, basting lamb with pan juices every 1/2 hour, until a meat thermometer inserted into thickest part of meat reads 130 degrees, about 1 3/4 hours total.
Remove from oven, tent loosely with foil, and let rest 15 minutes before carving.