Chop onions and fry in oil until slightly golden then add the turmeric to it. Fry chicken in onions until color changes. Add 1/2 - 1c water and bring to boil. Turn heat down and let boil slowly until chicken is cooked.
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 optional eggs to the yogurt and mix very well.
Wash rice and pour 3c water in a large pot and bring to boil. Pour in rice and cook while stirring occasionally until rice is al dente. Drain. (Basmati will cook a lot quicker than regular rice).
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 1.5 hours on medium low heat. Note that the longer Tah-Chin is cooked, the thicker the TahDig (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.