For making Yakhni : First take a deep thick bottomed handi in which you want to make biryani. You can either directly cook in the same handi or you can prepare yakhni separately and assemble in a different vessel. I prefer doing it separately.
So in a thick bottomed broad base vessel also known as a handi, heat oil till piping hot.
As soon as the oil is hot, add shah jeera and the other whole spices immediately.
The moment those spices starts to pop and splutter, add ginger- garlic paste and saute.
Now add coarsely ground onions and mix well. Fry all together till they change their color and raw smell goes off.
Now add the mutton pieces and sour curd with slit green chilies, grated coconut and mix all well. After a while you will see the curd leaving water. This is when you have to cook the mutton till it goes tender.
Add chopped mint and coriander, mix well and close the lid. Let the mutton cook now in slow flame till it becomes soft and tender. This may take 30-35 mins or even more than that. Depends on the quality of the mutton and size of the pieces.
Once the mutton is done, add coriander powder and salt as per you taste and fry it all nicely in high flame. You can now see oil oozing out from here and there. Just make sure the oil is sufficient for the yakhni. It should not be less.
The consistency of the yakhni should not be too dry. Turn off the gas and keep the yakhni aside.
Long grain rice are the best for making biryani. You can use them directly but i would suggest you to soak the rice 1 hour before cooking biryani. This enhances the size of the long grain and also increases the quantity of the rice used.
Take another thick bottomed vessel in which you wish to cook the rice. Add water in double quantity of rice and boil it.
While the water boils, add 1 tbsp of lemon juice and 1 tbsp ghee and salt in the water before adding rice.
Once the water starts boiling, add rice into it and cook till the rice is 3/4th cooked. You can check it by pressing the grain with your finger. Well here the rice is kept 1/4th raw because that will be cooked after assembling the biryani.
When the rice is done, turn off the gas and immediately drain water of the rice through a strainer and spread it on a big tray to avoid getting sticky.
Take a thick flat iron tawa and place it on the gas and heat. Place another thick bottomed vessel over the tawa.
Sprinkle some curd all over the bottom to avoid the yakhni getting burnt during dum.
Spread yakhni all over the bottom of the vessel evenly.
Sprinkle some mint and coriander over it.
Now spread the semi cooked rice all over and cover it completely.
Now add lemon juice on the rice all over, spread saffron milk (soak pinch of saffron in 1/2 cup milk for 30 mins in fridge).
Sprinkle kewda water and spread slit green chilies here and there.
Now add chopped mint, coriander and lots of fried onions and seal the vessel with aluminium foil properly from all sides.
Place the lid on top of the foil cover and keep a heavy pan on it such that the heat gets no place to come out.
Keep the biryani on dum in slow flames for around 20-25 mins.
Once the biryani is done, remove the foil and cut the rice from one side and shuffle it gently such that the rice and the yakhni mix evenly and the rice also does not break.
Serve hot with your favorite raita and salad.