It may be an intimidating protein, but if you need to feed a crowd, lamb's the way to go. Orange and lemon in the marinade make for a bright counterpoint to the earthy cumin. Stuff leftovers into pitas, and drizzle with yogurt.
Combine the first 11 ingredients in a small bowl, stirring well. Place lamb in a roasting pan; rub with garlic mixture. Cover with plastic wrap, and refrigerate for 8 hours or overnight.
Remove lamb from the refrigerator. Sprinkle lamb with 1 teaspoon salt and black pepper. Place lamb on the rack of a roasting pan coated with cooking spray, and place rack in pan. Let lamb stand for 1 hour at room temperature.
Preheat oven to 425°.
Roast lamb at 425° for 30 minutes. Reduce oven temperature to 375° (do not remove lamb from oven); bake an additional 45 minutes or until a thermometer inserted into thickest portion of roast registers 135° or until desired degree of doneness. Let stand for 20 minutes; slice.