Preheat your oven to 325°F (165°C).
In a small bowl, mix the minced garlic, ground cumin, ground coriander, ground cinnamon, ground cardamom, ground black pepper, salt, and olive oil to form a paste.
Rub the spice paste all over the lamb leg, ensuring it is well-coated.
Place the quartered onions in a large roasting pan and set the spiced lamb leg on top.
Add the water or lamb broth to the roasting pan.
Cover the pan tightly with aluminum foil and roast in the preheated oven for about 3-4 hours, or until the lamb is tender and falls off the bone.
While the lamb is roasting, prepare the spiced rice.
In a large pot, heat the vegetable oil or ghee over medium heat.
Add the chopped onion and sauté until golden brown, about 5-7 minutes.
Stir in the minced garlic and cook for another minute.
Add the ground cumin, ground coriander, ground cinnamon, ground cardamom, ground turmeric, and ground black pepper. Cook for 1-2 minutes until fragrant.
Add the soaked and drained basmati rice to the pot and stir to coat the rice with the spices.
Stir in the raisins and slivered almonds.
Pour in the chicken broth or water and add salt to taste.
Bring the mixture to a boil, then reduce the heat to low, cover, and simmer for about 20 minutes, or until the rice is cooked and the liquid is absorbed.
Once the lamb is cooked, remove it from the oven and let it rest for a few minutes before carving.
Enjoy your meal!