Soften the chillies. Soak the dried red chillies in hot water for at least 30 minutes.
Give the fish its first coating. Pat the cleaned fish dry, then coat it with a little lemon juice, turmeric and salt. Keep it refrigerated while preparing the masala.
Prepare the spices. Gently roast the cumin and coriander seeds until aromatic. Grind them with the soaked chillies, peppercorns, fenugreek, curry leaves, garlic, ginger, green chilli, tamarind, turmeric and salt. Add the optional vinegar if using it, and only enough water to get a smooth, thick paste.
Divide the paste before marinating. Set half of the freshly ground masala aside in a separate bowl for the final coating. Use the other half to coat the fish. Add a little water if needed so it resembles a thick batter and clings to the pieces. Cover and refrigerate for 1 hour. Keep the reserved paste separate from the raw fish.
Shallow-fry in coconut oil. Heat coconut oil in a tawa or wide frying pan and add some curry leaves. Shallow-fry the fish over moderate heat, turning carefully, until cooked through. Cook in batches if needed and lift the fried pieces out gently. The timing will depend on the fish and thickness of each piece; the centre should be opaque and separate easily with a fork.
Cook the reserved masala. Add a little more coconut oil to the same pan, followed by curry leaves and the reserved paste. Add a little water to loosen it to a thick, spoonable coating. Cook, stirring, until the rawness is gone and the masala is fragrant. Keep the heat gentle enough that the paste does not scorch.
Slather and serve. Place the fried fish over the cooked masala and carefully spread the coating over each piece. Handle gently so the fish stays intact. Serve hot, with Neer Dose to enjoy all that lovely masala.