For preparing keralan prawn biriyani, first mix together the powders for marinade together with the prawns.
You can skip the 20 minutes marination time but this step packs the prawns with flavour. If using frozen prawns thaw them first as this helps the marinade enter the prawns. I prefer raw prawns since the cooked ones wont absorb flavour and tend to go rubbery on further cooking.
Now the 20 minutes cooking time starts. Pour 5 cups of water in a microwavable pot and add 2 1/2 cups of rice to it. This needs to be cooked on full power for 20 minutes.
In a non-stick pan add 3 tbsp of oil and saute the onion for rice till soft. This way the rice remains white after cooking. Do not worry if tips of some slices turn a bit brown. Your biriyani would still taste superb. Take the rice pot from microwave and add the sauteed onion with salt to taste and resume cooking for the remaining time.
In the same pan, shallow fry the marinated prawns avoiding pile up. I used a 20 cm diameter deep pan which takes 20-22 tiger prawns at a time, so your can do 500g prawns in 2 batches.
Fry one side for 4-5 minutes, turn sides and fry for another 1 minute. This way the prawns remain juicy inside. Use a slotted spoon to transfer them to kitchen towel to mop up oil.
In the same oil saute the onion for prawn masala till light brown and soft.
Add ginger paste, garlic paste and green chillies and saute. Depending on your fiery threshold you can either add more chillies or take the seeds out of the chillies.
Now add in order coriander powder, 1/2 cup water, salt, fried prawns, lemon juice, leaves and 1 1/2 tsp garam masala. Your prawn masala is ready. Now this can be used as an accompaniment to flat bread too if you like. I use coriander seeds instead of powder(toss them dry in a pan, let them cool down and coarse grind using pestle and mortar).
You will see that there is still more time for the rice to cook. If brave enough(to add on a few more calories), use this time to fry some thin sliced onions, cashew nuts and raisins to use as garnish. Shop brought fried onions are a big no-no, they seem to taste of the weirdest oil ever. This may spoil all your hard work.
Once the 20 minutes is up, you will find that all the water is soaked up and the rice is dry on top. Use a fork to fluff up rice. Now you have two options. Either go traditional and layer rice and prawn masala in alternating layers together with the garnish and chopped coriander leaves in between layers and oven bake or microwave for another 10-12 minutes.
I go for the easy option of lightly mixing the two together.
To serve, transfer to a flat 5-6 cm deep serving plate and garnish with roasted cashew nuts, raisins and fried onions. Serve with yogurt raitha, hot pickle and pappadoms.