Marinate the prawns with salt, turmeric and 1 tbsp mustard oil. Keep aside for 10 minutes.
Blend mustard seeds, 2 green chillies, pinch salt and a little coconut water into a smooth mustard paste.
Roughly chop and gently mash lightly with a spoon. Do not blend.
In a bowl, combine, marinated prawns, mustard paste, tender coconut flesh, thin fresh coconut milk, remaining mustard oil, slit green chillies, a pinch of sugar. Mix gently and let it rest for 5–10 minutes.
Scoop out the soft flesh from the tender coconut ligthly. Pour the mixture into the emptied tender coconut shell with scrapped flesh. ( Ask the coconut seller to cut open the top portion neatly and keep it aside to use later as a lid.)
Seal the top tightly with foil or atta dough.
Steam on gentle medium heat for 30–35 minutes or bake in a pre-heated oven at 180°C for 30–35 minutes.
Rest for 5 minutes before opening. Finish with a few drops of raw mustard oil and serve hot with steamed rice.
