Slow roasting the lamb is the secret to its tenderness. Traditionally, the meat is baked in a clay oven or over an open fire, but a similar result can be achieved in modern settings by long baking in the oven at a low temperature. Butter and Ras el Hanout spice (if available) give the meat an unforgettable aroma.
Lamb in Mauritania is served simply, often with couscous or just fresh bread and herbal tea.
In a small bowl, mix the softened butter, ground cumin, Ras el Hanout, turmeric, minced garlic, salt, and pepper to create a spiced butter.
Pat the lamb leg dry with a paper towel. Make several deep cuts into the meat with a sharp knife.
Rub the spiced butter thoroughly into the meat, especially into the cuts. Marinate at room temperature for 1 hour, or in the refrigerator overnight.
Preheat the oven to 165 °C (329 °F).
Place the meat in a roasting pan. You can place a small rack underneath to catch the drippings. Cover the roasting pan with aluminum foil to trap the heat and allow the lamb to steam in its own juices.
Roast for 3.5 to 4 hours (or until the internal temperature reaches 90 °C (194 °F) and the meat is easily pulled apart with a fork).
For the last 30 minutes of roasting, remove the aluminum foil and increase the temperature to 200 °C (392 °F) to create a golden-brown and crispy crust.
Remove the meat from the oven and let it rest for 15 minutes, loosely covered with aluminum foil, to allow the juices to redistribute.
Serve the whole leg, which can be easily broken into pieces, along with the pan juices and traditionally with couscous or fresh bread and a simple salad side dish.