Cut 4 large slits in the lamb leg on both sides.
In each slit, add one garlic clove and about 1/4 tablespoons of black pepper.
In a roasting pan or a large baking dish with sides, place a large piece of foil, use two if necessary.
Place parchment paper inside the foil. These will wrap around the leg so make sure to use enough to cover the leg from all sides. Nest as many pieces of foil and parchment as you want.
Place the lamb leg on the parchment paper.
Pour the onion mixture over and rub it all around the leg. Tightly wrap the parchment paper over the leg, then wrap the foil to close tightly. Let it marinate until you heat up your oven to 550F.
Add two cups of water to the pan. And bake for 30 minutes.
Reduce heat to 350F and continue cooking until done to your liking.
Uncover and broil the top slightly.
Let it rest for 15 minutes before carving and serving.