To brine the pork, for every 4 cups of water, you add 1/4 cup of salt and 1/8 cup of sugar. Brine the pork for 2-4 hours.
When ready to start prep, remove the pork from the fridge and bring up to room temperature.
In the meantime, make the tonkatsu sauce by combining all the ingredients in a bowl for serving. This sauce can be kept in an airtight container in the refrigerator for a couple of weeks.
Heat the vegetable oil – a little more than the depth of the fillet – to 170°C (340°F) in a deep frying pan.
Slice the cabbage as thinly as possible and set aside.
Place the flour, the beaten eggs and the panko breadcrumbs in three wide bowls adjacent to each other. Have an oven baking tray ready to place the fillets on before frying. Add the salt and pepper to the flour and mix in well.
For the next step, use one hand for the wet ingredients (the eggs) and one for the dry. Coat the first pork fillet with the flour and shake off any excess. Coat in the beaten egg and hold the fillet above the egg mixture until it stops dripping. Coat the fillet with the panko and firmly press the panko into the egg coating. Don’t forget to coat the sides and the end of each fillet. Repeat for all the fillets.
Cook the pork fillets in batches, being careful not to overcrowd the frying pan. Note that if you have a wide frying pan and can do four fillets at a time, the temperature might drop when you put the fillets in. Adjust your heat as necessary.
Using a kitchen timer, turn the fillets once every minute to get a golden coating of panko crumbs. The fillets should be cooked after four minutes. Double check with a thermometer, they should be at 57°C to 61°C (134°F to 140°F). When they are cooked, transfer to a plate with a paper towel and keep warm before slicing.
Slice the pork on an angle into 2cm-wide strips. Place each portion on a plate, along with the sliced cabbage and tonkatsu sauce on individual small bowls for dipping or pouring.