Grind together chopped mint and coriander leaves (1/3 cup each), yogurt and soaked cashew nuts (after draining) to a smooth paste. keep aside.
Heat 1 tbsp ghee and 2 tbsp oil in a deep and wide pan. Add the whole spices listed under Prawns Masala (Cinnamon - Bay leaf)
Add sliced onion (5). Cook till it becomes brown. Add crushed ginger, garlic and green chilli. Cook till the raw smell goes, 3-4 mins. Add all masala powders (from chilli powder - garam masala). Fry for 2-3 mins, till oil starts appearing.
Add yogurt paste. Cook for 4-5 mins. Add chopped tomatoes. Cook till it becomes soft. Add salt and cleaned prawns. Mix well.
Cover and cook, till prawns is done and water is dried (12-15 mins). No need to add water while cooking prawns. Water will be released from the prawns.
Rice Preparation:
Rinse the rice under running water 3-4 times. Heat ghee in a wide and deep pan. Add whole spices listed under "for rice" (from cardamom - cinnamon).
Add chopped onion and cook till it becomes soft, no need to brown it. Add Crushed ginger & garlic. Cook for 3-4 mins. Add rice and salt and gently mix. Fry the rice for 3-4 mins. Add water and lemon juice. Bring to boil.
Simmer and cook till rice is done and water is dried. Refer notes for detailed instructions.
Layering the biriyani:
Grease a deep and wide oven proof dish with ghee. Spread a layer of rice, sprinkle chopped mint and coriander , fried onions (optional) and drizzle some ghee on top of it. Spread a layer of prawns masala, followed by rice and toppings (mint, coriander leaves, ghee & fried onion). Repeat the layering process, until the rice and masala is used completely. The top layer has to be rice. Put a wet kitchen towel on the top layer. Cover it with a tight lid.
Preheat oven to 200 C, 10 mins before baking. Bake in the pre heated oven for 20 mins. Take out of oven and let rest for another 10 mins. Open and serve. Garnish with fried onion, cashew nuts, mint and coriander leaves.
If you dont have an oven, check out the "notes" section here to see, how to finish cooking biriyani on stovetop.
Notes
Cooking of rice: Though the usual ratio of rice to water is 1:2, I find it works better when I add 1/2 cup less than the required qty. Here for 2.5 cups of rice, I used 4.5 cups of water instead of 5. Also, once the rice is cooked, I spread it out on a big tray and cool it before layering. I find that it helps the rice from getting overcooked or mashed up during baking. If you are cooking this for a party, you can make the prawns masala 1-2 days in advance and refrigerate.