To make the burgers, rinse the prawns in cold water, then pat dry on kitchen paper. Tip half the prawns and the spring onion into a food processor with a generous pinch of salt, and pulse to a rough paste. Add the egg white and cornflour, pulse a few more times then add the rest of the prawns and pulse to roughly chop them into the mixture.
Tip the breadcrumbs into a shallow dish or onto a plate. Scoop out half the prawn mixture with your hands and shape into a burger as well as you can (the mixture will be sticky). Press the burger into the crumbs, then flip over and pack crumbs around the sides so it’s completely coated. Repeat with the remaining mixture and crumbs, then place the burgers on a plate, cover with cling film and chill until ready to cook.
Next, mix together the chilli mayo ingredients in a bowl. For the slaw, put all the ingredients in a bowl, season, then toss to combine. You can make the burgers, slaw and chilli mayo a day ahead and chill them.
To cook the burgers, heat about 2cm oil in a frying pan until just starting to shimmer and a breadcrumb dropped in sizzles and browns. Very carefully fry the burgers for 3-4 mins each side until golden, crispy and springy to the touch – you may need to turn them a few times to cook them all the way through. Carefully lift out of the oil onto kitchen paper to drain. Cut the buns in half and toast the cut sides under the grill. Assemble the burgers by spooning the slaw over the bottom of each bun, adding a burger and topping with the mayo. Serve straight away with extra chilli mayo, if you like.