Cut off excess fat from lamb. Chop lamb into 60 cubes. Add lamb, 3 tablespoons molasses, onion, salt, and pepper to a resealable plastic bag and chill 8 hours, flipping bag a few times. Take lamb out of bag and dispose of liquid.
Thread 4 lamb cubes and 4 onion pieces alternately onto each of the 15 kabob sticks.
Arrange kabobs on a broiler pan and broil 3 minutes per side. Spoon 3 tablespoons of molasses over kabobs. Serve with lemon slices and garnish with oregano leaves, if desired.