Wash the lentils thoroughly and drain them. Wash the raisins and pat dry on kitchen paper. Take the stones/pits out of the dates and chop them to the same size as the raisins. Peel the onion and chop it finely.
Wash, peel and thinly slice the potatoes, if using, and put them in a bowl of cold water to prevent discoloration.
Cooking Put the lentils in a saucepan and add about 500 ml/just under 1 pint of cold water; do not add salt. Bring to the boil and cook on a medium heat until the lentils are al dente. Drain and set aside.
Heat 30 g/1 oz of the butter and 2 tablespoons of oil in a frying pan/skillet on a medium heat and fry the onion until golden.
Reduce the heat and add the raisins and dates and stir. Add the cooked lentils and 1 teaspoon of turmeric. Add salt to taste. Stir and set aside.
Pour 400 ml/14 fl oz of water into a heavy-based saucepan and bring to the boil. Drain the rice and add to the boiling water with 1 tablespoon of salt, 50 g/2 oz of butter and 3 tablespoons of oil. Leave to simmer, uncovered, on a medium heat, until all the water has been absorbed and holes start to appear on the surface of the rice (approximately 10 minutes).
Tip the rice into a shallow dish. (To make a rice tahdig (instead of potato), set aside 2 heaped tablespoons of the plain rice.) Gently fold the lentil and raisin mixture into the rice and add 3 tablespoons of the liquid saffron take care not to break the grains of rice as you mix.
If you are using potatoes for the tahdig, drain and dry the potato slices. Return the saucepan to the heat. Add 2 tablespoons of vegetable oil and when it is sizzling arrange the sliced potatoes in a layer to cover the bottom of the pan. (Alternatively, you can use a couple of tablespoons of cooked plain rice instead of potato slices, or make other types of tahdig.)
Spoon the lentil and rice mixture over the layer of potatoes. Keep the rice in a pyramid shape and away from the sides of the pan as much as possible. Dot the rest of the butter on top of the rice. Wrap the lid in a clean tea towel and place it firmly on the pan. Leave to steam on a low heat (with a heat diffuser on a gas flame) for 50 minutes to 1 hour.
When the rice is ready, dip the bottom of the pan in cold water, or stand the pan in 5 cm/2 inches of cold water for few minutes, to help release the bottom layer. Serve the rice in a shallow dish, fluffing it as you spoon it out. Drizzle the remaining liquid saffron over the rice. Detach and break the crispy layer of potatoes or plain rice from the bottom of the pan and serve on a separate plate.
To serve, accompany with a dish of fresh herbs, radishes and spring onions/scallions. Mango chutney or pickles also go well with this dish.
Preparing the tahdig During the steaming process a crust will form in the bottom of the pan. This is called the tahdig and is a delicious accompaniment to the rice, much prized by Iranians. You can adjust the thickness and flavor of your tahdig by adding various ingredients: see the list below. Before returning the boiled rice to the saucepan, heat a mixture of butter and vegetable oil in the pan (just enough to cover the base of the pan) on a medium heat until sizzling. Then cover the bottom of the pan with a layer of your chosen ingredient. Carefully spoon the rice on top, then steam.
If you are making tahdigfor a polo, add all the polo ingredients of your choice to the rice before beginning the tahdig process.
Tips: Lentils absorb a lot of oil, so you might need to add more butter to the rice when serving, if you find the rice to be dry. Do not use rice mixed with lentils for the tahdig: the lentils turn into hard pellets in the oil and the raisins and dates burn.