Pat the fish fillets and prawns dry on kitchen paper. Rub the turmeric, onion seeds and half the salt over the fish and set aside for 15 minutes.
Heat a tablespoon of the oil in a frying pan, add the prawns and sauté for 30 seconds, until they turn pink. Mix in the chilli powder and the remaining salt and leave to cool.
Bring a large saucepan of water to the boil, add the spinach and stir it around in the water for 30 seconds, until just wilted. Drain well, refresh in a bowl of ice-cold water, then drain again and dry on kitchen paper.
Lay one of the fish fillets out on a work surface and arrange some of the spinach leaves on top to cover it.
Sprinkle with garam masala and place half a prawn at the tail end of the fillet, then roll it up towards the head end. Secure with a bamboo skewer and repeat with the rest of the fillets.
To prepare the sauce, heat the oil in a frying pan, add the onion, ginger, garlic and green chillies and sauté till the onion is soft. Add the cashew nuts and sauté until they turn golden.
Add the coriander and mint and sauté again until softened. Remove from the heat and leave to cool, then blend to a smooth paste in a blender or food processor, adding a little water if required.
For the tempering, heat the oil in a frying pan and add the mustard seeds and red chillies. When the mustard seeds crackle and the chillies change colour, add the sauce and bring to the boil. Season with the salt and sugar, add the lemon juice, then remove from the heat and keep warm.
To cook the fish, heat the remaining 3 tablespoons of oil in a large, non-stick frying pan, add the rolled-up fillets and fry them on all sides for a couple of minutes. Transfer them to a baking tray, dot the butter over the top and bake in an oven preheated to 180°C/350°F/Gas Mark 4 for 4–6 minutes.
To serve, spoon a couple of tablespoons of the sauce on to each plate and place 2 rolls on top.
