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Tip 300g of the prawns into a food processor with the cod, coriander, chilli flakes, garlic, ginger and 1 tsp salt. Pulse until the mixture comes together, then tip out into a large bowl. Roughly chop the remaining prawns and stir these through the prawn and cod mixture. Form the mixture into four even-sized patties using damp hands. Cover and chill for 1 hr.
For the pickles, tip the onions and radishes into a non-metallic bowl with the lime juice and a pinch of salt, then set aside to pickle for 30 mins.
Meanwhile, make the guacamole. Scoop the avocado flesh into a bowl and gently mash (try to retain some chunkiness) with a fork, then fold in the coriander, lime juice and a large pinch of salt. For the chilli mayo, mix the mayonnaise and hot sauce together. Set both aside until ready to use.
Light the barbecue and wait until the coals are ashen, then cook the burgers for 3-4 mins on each side until golden and cooked through. Alternatively, heat the rapeseed oil in a large frying pan over a medium-high heat. Dust the burgers in the cornflour, then fry for 4 mins on each side until golden and crisp. Keep warm on a plate. Lightly toast the cut side of the buns on the barbecue or in the pan, then top with the burgers, guacamole, lettuce, pickled onions and chilli mayo.