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Heat oven to 200C/180C fan/gas 6. Mix the onions, ginger, oil, 2 tbsp curry powder, the mustard seeds and 1 tsp of the onion or nigella seeds with 5 tbsp water in a baking dish roughly 25 x 18cm. Roast in the oven for 10-15 mins until the onions are softened.
Stir in the lentils, split peas or chana dhal lentils, 1 tsp of the turmeric, the coconut milk and half a can of water, and return to the oven for 30 mins. Meanwhile, mix together the remaining turmeric, onion seeds and curry powder and the yogurt. Carefully slice the skin off the fish fillets and place on kitchen paper to dry, then rub the yogurt all over the fish fillets and leave to marinate in the fridge while you cook the lentils.
Give the dhal a good stir, mix in the tomatoes and juice from 1 lime, plus 1 tsp salt. Sit the fish fillets on top with a sprinkling of extra seasoning. Return to the oven and cook for a further 15 mins until the fish is done. Remove the dish from the oven and turn on the grill. Place the fish skin on a baking tray, sprinkle with some salt and grill, turning, until crispy. Snap into pieces and scatter over the fish with some coriander and the crispy onions. Serve with more yogurt, lime wedges, naan bread and mango chutney.