Pre-heat oven at 200c on the roast/baking setting.
Add all of the "Chicken Marinade Paste" ingredients to a food processor and grind together. Empty into a bowl.
Now add all of the "Additional Ingredients for Chicken" to this bowl and mix well. Allow to marinade - anywhere from 2 hours up to 24 hours ahead ideal. Remove from fridge an hour before cooking to allow it to get to room temperature.
To make the rice, bring a large pot of water to boil (Approx 4litres). Drain the soaked rice. When the water comes to a boil, carefully add all of the remaining "For the Rice" ingredients to the water.
Mix well and allow the water to come back up to the boil. Reduce to a rolling simmer and mix the rice every minute or so. Cook for 3-4 minutes or until the rice is par cooked. It should be al dente and you be able to just break a grain of rice with your nail.
Be careful not to overcook the rice - it should be about 50% cooked. Drain immediately in a colander. Keep the drained rice covered to ensure it stays warm and then keep aside.
To make the saffron milk, bring the milk and cream along with the salt and turmeric to boil in a small sauce pan. As soon as it boils, turn off the heat. Add the saffron strands along with the kewra water. Allow the saffron to infuse well (you should see the milk turn yellow). Keep aside.
Now it's time to layer! Tip in the marinaded chicken to your ovenware - make sure it is spread out nice and flat so the base is not overcrowded. Then add the rice to cover (there should be about 50% rice and 50% chicken ratio). Then sprinkle over the mint, coriander and crunchy golden onions.
Finally, evenly drizzle the saffron milk all over the biryani.
Now cover the biryani with your puff pastry sheet. Make sure you spend some time to seal the pastry around the edges - this is really important. There should be some overhang but if there is a lot of excess around the sides, you can cut some off.
If you are not using pastry, ensure that your ovenware has a tight fitting lid.
Place in the oven and cook for 15 minutes. Then reduce the temperature to 180c and cook for a further 15-20 minutes.
When you remove the biryani from the oven, the puff pastry should be crisp and golden. Use a serving spoon to cut a hole around the edge of the pastry lid.
Then using the spoon, mix together all of the layers. Serve onto a platter. If you like, you can garnish with some more crunchy fried onions and coriander.