Place the kingfish in a food processor and pulse until very finely chopped.
Heat the peanut oil in a large frypan over a high heat. Add the onion and garlic. Cook for 1-2 minutes until slightly softened and then add the fish and cook, tossing, for 1-2 minutes until starting to colour.
Add the soy sauce, mirin, oyster sauce and fish sauce. Stir through and cook for 2-3 minutes until the sauce has reduced.
Serve the mixture in lettuce leaves topped with cabbage, radish slices and tonkatsu sauce.