Split 8 prawns for the toast and set aside. Finely chop 4 prawns and place in a bowl with the ginger, salt, sugar, rice wine, sesame oil, soy sauce, and spring onions. Leave to marinate for 10 minutes.
In a food processor, pulse the fish, then add the egg white and pulse until smooth. Add the cream, mix, then add to the chopped prawn and stir thoroughly.
Preheat a deep-fryer to 170°C, or heat 8 cm oil in a pot and check temperature with a thermometer.
Mix the white and black sesame seeds together and tip onto a plate or flat bowl. Smear the slices of bread with a tablespoon of prawn mixture and firmly press down. Top with 2 pieces sliced prawn. Dip underside of bread into the whisked egg white and dip in the sesame seeds. Flip the slice to coat the prawn side with sesame seeds too. Fry bread with the prawn side down for 2 minutes then flip it and fry the underside for 3 minutes. Drain really well on kitchen paper.
Squeeze over the lemon and serve with a dollop of mayonnaise, and sprinkle with shiso leaves and chilli.