This large roast is great as a Sunday dinner centerpiece, and the mild seasonings encourage leftovers to blend into a variety of dishes. For a nice accompaniment, toss halved fingerling potatoes with olive oil and sea salt. Place them on the broiler pan with the lamb during the last half-hour of cooking.
Place first 10 ingredients in a food processor; pulse until finely chopped. Place onion mixture and lamb in a large zip-top plastic bag. Seal bag; rub onion mixture into lamb. Marinate in refrigerator for 4 hours to overnight, turning occasionally.
Preheat oven to 450°.
Remove lamb from bag; discard marinade. Place lamb on a broiler pan coated with cooking spray. Bake at 450° for 15 minutes. Decrease oven temperature to 350° (do not remove lamb from oven). Bake at 350° for 1 hour and 20 minutes or until a thermometer inserted in thickest portion of the lamb registers 145° (medium-rare). Place lamb on a cutting board. Cover loosely with foil; let stand for 15 minutes.