Cut off an inch from the top of the tender coconut, drain the water and scoop out the flesh. Retain the top to act as a lid.
Preheat oven to 220°C. Add salt and turmeric powder to the prawns. Mix and set aside. Heat mustard oil to smoking point. Cool slightly and add paanch phoron. When the seeds splutter add onions and sauté. Add ginger and garlic pastes, green chillies and coconut and continue to sauté for two minutes. Add one cup of water and stir. Add prawns and stir. Add coconut flesh and adjust salt. Transfer the mixture into tender coconut shell. Cover with the lid and seal with dough. Place in the preheated oven and cook for about twenty minutes.
Open the lid and serve hot.