Combine the ingredients under "for marination" in a pressure cooker and leave to rest for an hour.
Cook on high flame for one whistle and switch off. Allow the pressure to go by itself.
In a saucepan, heat oil. Saute the onions and green chillies till the onions are wilted.
Add the ginger garlic paste and cook till raw smell is gone. Add the tomatoes and cook on low flame, stirring occasionally till the masala gets all mashed up.
Add the turmeric and red chilli powder and cook for another couple of minutes.
Add the cooked prawns along with its liquid into this masala and cook till the sauce thickens.
Add in the fennel powder and garam masala powder and cook for another couple of minutes. Adjust the salt, add the coriander and mint leaves. Switch off and reserve.
In a deep saucepan, boil lot of water with the whole masalas and salt.
Add in the drained rice and cook on high flame till the water comes to a rolling boil.
Allow to cook for around 10 minutes on meduim heat or till the rice is almost 80% done - ie, when you bite in a rice, there will be a little raw portion in the center. Drain immediately and reserve.
In a fry pan, heat 2 tbsp ghee and fry the onions till nicely browned.
Fry the cashewnuts and raisins along with it. Reserve.
Heat a tawa for the dum. In a deep saucepan, put the remaining ghee at the bottom.
Put in the prawn masala. Sprinkle half the fried onions, garam masala powder, mint and coriander leaves.
Put in the rice and sprinkle the remaining of the dum ingredients on top.
Cover tightly with aluminium foil and close with the lid. Keep on the heated tawa on meduim-low flame for around 15-20 minutes.
Switch off and keep undisturbed for another 10 minutes.
For serving, take out the aluminuim foil. Mix well with a long spatula. Serve hot with cucumber onion raita and some thenga chammanthi!