Chettinad Mutton Biryani

Ingredients

Mutton - 1/2 KG
Onions - 3 large (sliced thin)
Green Chillies - 3 (slit in half)
Tomatoes - 2 large (sliced)
Garlic cloves - 5 (crushed)

whole spices
cloves - 5nos
pepper corns - 1 tsp
Cinnamon sticks - 2 inch
cardamom - 5 nos
1 bay leaves

Marinate the mutton for 1 hour ...
1/2 cup curd, 
2 tsp ginger-garlic paste,
1 tsp chilli powder
1/4 tsp turmeric powder
1 tsp garam masala powder
salt to taste

Soak 1 and 1/2 cups rice for 1/2 an hour and drain.(may add a half cup more)

Brown the sliced onions, keep aside half for later. 
In a pressure cooker, heat 1 tsp oil and add the whole spices..once they release the flavour, add the brown onions, add just the mutton pieces and slightly cook them, add the tomatoes, green chillies and the crushed garlic, add the marinade and pressure cook the mutton..I waited for 3 whistles.

Lightly fry 2 tsp coriander powder and keep aside

Once all the steam settles, empty the mutton into a vessel. Layer the remaining browned onions and half the rice add the mutton pieces ,the coriander powder and add the rest of the rice. Now take the gravy and measure it with the same cup/measure as the rice. Liberally add mint leaves throughout the layers. Add boiled water to make the liquid nearly double the quantity of rice. (I put in 2 and 3/4 of liquid)
I have found that a little less than double is always better, especially in a cooker.

Instead of boiled water, add warm milk with a pinch of saffron for added flavour
Two whistles, and wait 20 minutes before removing the lid.