Place half the prawns in a food processor with the spring onion. Pulse a few times until it is a rough paste.
Add the cornflour, Ginger, garlic and the egg white. Blitz until just combined.
Finely chop the remaining prawns and stir through the mixture with a teaspoon each of salt and white pepper.
Using wet hands (the mixture is sticky) form the mixture into 6 patties.
Put a large handful of panko breadcrumbs and the sesame seeds on a plate.
Coat each patty in the breadcrumbs and seeds. Cover with cling film and chill until ready to cook.
Heat a couple of cm of sunflower oil in a large frying pan. Heat to 180C. Fry the patties on each side for around 2-3 minutes until golden brown all over, then drain on kitchen paper.
Meanwhile finely slice the white cabbage and mix with the juice of 1/2 lemon and a little salt.
Make the sauce by mixing together about 5 tablespoons of mayo with 2 tablespoons sriracha (more if you like it hot) and a tablespoon of ketchup.
The bao I bought just needed 1 minute in the microwave.
Assemble the Prawn Katsu Bao Burger by adding a little cabbage to the bun, then a Prawn Katsu patty and a little sauce on top. Finish with some sliced spring onion.