1: Place the fish fillets and prawns on a rimmed, non-metallic plate. Sprinkle over the sea salt flakes, then place the fish in the fridge for 1½ - 2 hours until the textures become firm.
2: After the fish has become firm, rinse it thoroughly under running cold water, drain well on a tea towel and pat dry.
3: If you have a mincer, mince the fish into a bowl. If not, very finely chop the fish with a sharp knife. Add the garlic, capers, bicarbonate of soda, dill and mustard powder and mix together. Shape into 4 large burgers, cover with clingfilm and leave in the fridge for at least 1 hour before cooking.
4: Meanwhile, make the herb mayonnaise. Bring a saucepan of salted water to the boil and place a bowl of iced water in the sink. Add the chervil, parsley and tarragon into the boiling water and blanch for about 2 minutes until the herbs are soft. Drain well, then immediately plunge them into the iced water to stop the cooking and set the colours. Leave to cool completely, then squeeze them to remove any excess liquid.
5: Place the herbs, egg yolks, white wine vinegar and Dijon mustard into a food processor and blend. With the motor still running, slowly add the oil until it emulsifies and thickens. Season, cover and keep in the fridge until needed.
6: Place the tomato and onion slices on a plate and season. Leave for about 10 minutes to draw out some moisture.
7: When ready to cook, preheat the grill to high and toast the buns. Heat the rapeseed oil in a large non-stick frying pan over a medium heat. Add the fish burgers and fry for 2–3 minutes on each side until cooked through and lightly coloured. Remove them from the pan and cover with foil to keep hot.
8: Add the pancetta to the pan and fry for about 4 minutes on each side until they are crispy and browned.
9: Divide the tomato slices between the buns and then add the fish burgers, wilted onion and lettuce. Top with the pancetta and finish with the herb mayonnaise. Serve immediately.