To make the burgers, defrost and 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 that sticks together. 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 onto a plate. Take out half the prawn mixture and press into a burger-patty shape. Press the patty into the crumbs, ensuring you coat all over. Repeat with the remaining mixture and crumbs, then place the burgers on a plate in the fridge to chill.
Next, make the sauce by combining the chilli mayo ingredients. To make the coleslaw, put all the ingredients in a bowl, season, and mix. You can make the burgers, slaw and chilli mayo a day ahead and chill them to speed things up on the day.
To cook the burgers, heat about 2cm oil in a frying pan until just starting to bubble. To test, drop a breadcrumb in. If it sizzles and browns, it is hot enough. Be careful with the hot oil! Fry the burgers for 3-4 mins each side until golden, crispy and springy to the touch. Carefully lift out of the oil onto kitchen paper to drain. Toast the burger buns under the grill. Spread the slaw over the bottom of each bun, adding a patty and topping with the mayo. Serve straight away with extra chilli mayo, to taste.
Notes
This recipe would work equally well with lobster, cod or salmon to replace the king prawns. If sriracha is too spicy, try with a milder sweet chilli sauce instead.