How to Make It
Place lamb in a 2-gallon zip-top plastic bag. Add juice, oregano, 2 tablespoons cumin, and 2 teaspoons salt; rub over lamb. Refrigerate lamb overnight.
Puree orange juice, garlic, olive oil, and onions in a blender. Place in a bowl; cover and chill overnight.
Combine remaining 2 teaspoons cumin, remaining 1 teaspoon salt, and pepper in a small dish.
Preheat grill to medium. Remove lamb from refrigerator; let stand 30 minutes.
Place lamb on grill rack coated with cooking spray; cover. Grill lamb over medium direct heat for 1 1/2 hours or until a thermometer registers 135° (medium), turning and basting frequently with onion mixture. Remove lamb; let rest 15 minutes. (Internal temperature of lamb will rise to about 145°, or medium-well.) Thinly slice lamb; sprinkle with cumin mixture.