Heat 1 Tbsp oil until hot but not smoking in a large oven-safe pan or roasting pan (I use an
enamel-coated dutch oven). Place the lamb, fat-side down into the pan and sear for about 3 mins.
Turn the lamb over, scattering the onion pieces and garlic cloves underneath. Pour 1/2 cup water into the pan and cover tightly (use a lid or tightly wrapped foil).
Remove the lid and allow the lamb to roast for one more hour, uncovered. The lamb should be well-browned and fall off the bone tender at the end of the hour.