Soaking the rice: Clean the rice well, then rinse it several times and soak it in water for a few hours. If you're short of time (say 10 minutes), you'd better soak the rice in warm water.
Prepare the materials. Combine yogurt, egg yolk, saffron, oil, rosewater,cardamom and salt. Rinse the rice a few times so that it gets impurities removed.
Boiling the water: In a non-stick pot, pour some water and put it on the heat. The amount of water should be enough for two knuckles to be above the rice. Bring the water to a boil.
Pour the rice in the boiling water.
Add some salt. As for 3 cups of rice roughly one tablespoon of salt will be enough. Stir a bit so as not to let the rice grains stick together.
Draining the rice: Having half-cooked the rice, remove it from the heat and pour it into a drain. Rice grains ought not to be too soft, but rather firm. Do not put the lid on the pot. (cook until al-dente)As for testing, when you take a grain of rice and place it under your teeth, the inside should still be slightly firm while the outside is cooked. Never allow the rice to cook and soften completely since in the next step they're going to steam and soften nicely.As a general rule, depending on the quality of rice it takes 3 to 5 minutes to get half-cooked. Of course, the cooking time depends on the grade of the rice and the gas heat; some sooner, some later (and some never these days!). Pour the rice into the strainer.
Pour about 3 scoops of the rice into the ingredients ( yogurt, egg yolk, saffron, oil). How much rice to pour depends on the size of your pot and whether you want a thick or thin tahdig.But the important thing is that you should add as enough rice as not to get an overly loose and watery mixture. Put the pot on the heat and add two tablespoons of oil into it. Once it heats up a bit, pour the tahdig material into it and press with the back of a spoon or scoop so that it sticks well together and the surface gets flat.
Pour the remaining rice on it. Using a long toothpick or the scoop handle, make a few holes in the rice surface. The holes should get close to the bottom of the pot. This allows the steam to reach the bottom of polo well so that it can steam-cook uniformly. Wrap the lid in a clean towel and put it on the pot.Cook the rice over a gentle heat for about 40-60 minutes until it stews and the tahdig gets golden and crispy. If the heat is high, the tahdig will burn quickly because of saffron and its color will turn black.