Preheat your oven to 375°F (190°C).
Pat the lamb dry and rub generously with salt, black pepper, coriander, cumin, garlic, and ginger powders.
Drizzle with olive oil and the juice of one orange, massaging the marinade into the meat. Let it rest at least 30 minutes (or overnight in the fridge for deeper flavor).
Place the lamb in a roasting pan and cook for 60–75 minutes, or until the exterior is crisp and the inside is tender. For extra crispiness, sear the lamb in a hot skillet before roasting.
Slice and serve with thick orange rounds on the side, plus a helping of white rice to soak up the juices.