Scoop out the tender flesh from the coconut and reserve the shell.
Heat oil in a non-stick pan.
Add mustard seeds, curry leaves, peppercorns and sliced onion to the pan and saute.
Take prawns in a bowl. Add chilli sauce, mix well and set aside for 15 minutes.
Roughly chop dried red chillies and add to the pan, saute further.
Slice fresh turmeric and place in a grinder jar alongwith broken green chilli, coconut flesh, garlic cloves and roughly chopped ginger. Add very little water and grind into a coarse paste.
Add prawns in the pan alongwith ground paste, salt and mix well. Add coconut milk and mix well.
Transfer the prawn mixture into the reserved coconut shell, cover with the lid and seal with the wheat flour dough.
Heat sufficient water in a deep non-stick pan. Place the coconut shell in it and steam for 15-20 minutes.
Remove the seal and serve hot.