Soften the shallots, garlic and ginger with the oil and then blend with the vinegar and 2 tbsp water. Toast and grind the pepper, add to the paste.
Slash the turbot and rub in the spice paste. Allow to marinate briefly.
Sautee the onions in the remaining coconut oil, when softened, stir in the chillies, curry leaves and mustard seeds. Oil the foil and strew with coriander leaves and sesame seeds. Add half the onion and season with salt. Lay the fish on top and spread over the remaining onions.
Sprinkle with a little more vinegar and seal. Roast for 30 minutes at 200C and serve with boiled rice.