You need defrosted, thawed prawns for these. See notes on how to thaw frozen prawns. In case you have not kept the prawns for thawing the night before, just place the frozen prawns in a bowl of water and keep for 15 to 20 minutes.
At the same time, soak mustard seeds in water for 10 to 15 minutes.
After that, in a grinder, make a paste of soaked mustard seeds (without the water), 1 green chilli and 1/2 cup of coconut milk. You have to make a fine creamy paste. Best is to not use all of the coconut milk at a time but to pour it in installments and grind. In case you feel the need, then add a little more coconut milk and grind it till you have a fine smooth paste.
Now we make the base of daab chingri. In a mixing bowl, take the mustard paste and add the rest 1/2 cup of the coconut milk and keep aside. This is your base to which the prawns will be added later on.
Once the prawns have thawed, marinate the prawns with ginger garlic paste, little bit of salt, turmeric powder and the juice of one onion for 10-15 mins. To get the juice of one onion, all you have to do is grind one onion and strain it.
Take a frying pan and heat one tsp of mustard oil. Carefully slide the prawns coated with the marinade in the frying pan and sauté for a minute on both sides. Once done, take it out of the pan and transfer it to the mixing bowl which has the prepared sauce.
Add the rest of the mustard oil and salt if needed. Add a couple of slit green chilies.
Prepare the tender coconut by cutting the top of the shell. It should be a big enough mouth for the prawns and the sauce to go in. Discard the water or rather drink it up.
Now transfer the prawns into the empty shell. Cover the shell with the top of the tender coconut and seal it with an aata dough.
There are two way of cooking this. In the oven , you have to place the tender coconut for 45 minutes to an hour at 180 degree C. In case you want to make it on the gas, take a pressure cooker and add a little bit of water. Place the tender coconut inside the pressure cooker and cover and cook for one whistle.
Once the steam is out, let it cool down a bit and then take it out . Open the seal and serve Daab chingri with steamed rice and some more green chilies. While taking the sauce out, try and scrape out any tender coconut inside along with the sauce.