Place lamb, onion, salt and turmeric in a large saucepan. Cover with water, bring to boil.
Lower heat and simmer until the meat is falling off the bones, about an hour.
Remove lamb, and discard the bones.
Boil the lentils in water till half cooked and strain.
Wash the rice in water and soak while the lamb is cooking.
Strain and wash under running water to remove any remaining starch.
In a large bowl, add the uncooked rice, lamb, lentils and sultanas and stir carefully to distribute the ingredients.
In the bottom of a heavy based saucepan, or a Persian Rice cooker ladle about 1/2 cup of lamb stock, and cover with a layer of sliced, peeled potato to form the tahdig.
Gently spoon the rice mixture over the potato.
Measure out 4 cups of stock and gently pour over the rice mixture, being careful not to dislodge the sliced potato.
Place a tea-towel or a cloth under the saucepan or rice cookers lid to absorb any excess moisture while the rice is steaming.
Cook on a high heat until the sides of the pan are hot to touch, then reduce the heat to low and cook for about 30 – 45 minutes. Alternately if using a Persian Rice cooker, allow to run the full cycle.
Place the bottom of the saucepan or rice cooker pan in about 3 cm of water in a sink to loosen the tahdig.
Turn the pan upside down onto a round serving dish to serve.
A Persian rice cooker makes this dish much easier as the tahdig is formed perfectly each time without the risk of either soggy or burnt tahdig.