Trim fat from lamb. Cut lamb into 60 (3/4-inch) pieces. Combine lamb, 3 tablespoons molasses, grated onion, salt, and pepper in a large zip-top plastic bag; seal. Marinate in refrigerator 8 hours, turning bag occasionally. Remove lamb from bag; discard marinade.
Thread 4 lamb cubes and 4 onion pieces alternately onto each of 15 (6-inch) skewers.
Preheat broiler. Place kebabs on a broiler pan; broil 3 minutes on each side or until desired degree of doneness. Pour 3 tablespoons molasses evenly over kebabs. Garnish with lemon and oregano, if desired.