Preheat Oven: 300 °F, convection or 330 °F, standard. Place the leg of lamb on a cutting board, pat dry if needed. Mix the seasoning together in a small bowl and season the leg of lamb evenly on all sides. In a cast iron pan, over HIGH heat, brown the leg of lamb on all sides. Remove the leg of lamb from the heat and set aside.
Reduce the heat to Medium and add ghee. Brown the onions until slightly charred. Add tomatoes, garlic, turmeric, stirring occasionally until fragrant. Add water, continue stirring. Push everything to the outside, creating a pocket in the center.
Lay down slices of yams as a base for the leg of lamb.
Place leg of lamb over the base. Spoon some of the juices/sauce over the top of the lamb. Seal tightly with foil, this is important. Let no air in or out. Roast for 3-4 hours on the middle rack.
Check on your lamb, it should be tender to the fork check. Baste with the juices that have rendered which is now a combination of the original sauce and the rendered fat from your leg of lamb.
Remove from the oven and squeeze lemon over the top. Serve with your favorite sides.