Start by washing rice and soaking it in warm water (with added salt) for 2 hours. Then filter out the water.
Chop onions into thin slices and fry in oil until slightly golden. Wash and cut chicken, remove skin, and fry in onions until color changes. Add some water and bring to boil. Turn heat down and let boil slowly until cooked, adding more water if needed. Remove the bones.
While chicken is cooking, beat the yogurt until it is smooth. Dissolve saffron in half a cup of hot water. Add saffron, salt, pepper and egg-yolks to the yogurt and mix very well.
Pour a few glasses of water in a large pot and bring to boil. Pour in rice and cook while stirring occasionally until rice grows longer and slightly softens (Take care not to overcook the rice. It should still be too hard for eating). Again filter out the water.
Pour several spoons of oil and several spoons of the yogurt mix into a non-stick pot. Add a thin layer of rice and flatten using the back of a spoon. Add a layer of chicken on top followed by another layer of rice. Again flatten the rice. Spread several more spoons of the yogurt mix on the rice. Continue in this fashion until chicken, rice and the yogurt mix have been used up. Add some more oil on top. Put the lid on and cook for about 5 minutes over medium heat.
Place the pot in an oven (preheated to 250 F) and cook for 1.5 to 2 hours. Note that the longer Tah-Chin is cooked, the thicker the Tah-Dig (delicious crispy layer of rice at the bottom) will be. When cooked, remove the lid and let cool for a few minutes.
Place an inverted large dish over the pot and turn it over. Tap the pot in order to loosen the contents inside. The contents should fall on the dish in one piece with the Tah-Dig on the outside.