For pork: make small incisions around the edge of the pork, at 1.5cm intervals, so the meat doesn’t curl up when cooking.For chicken: you don’t need to this, move to next step.
Place the meat between two sheets of plastic wrap. Using a meat mallet or rolling pin, beat the meal until 1.5cm thick. Brush miso evenly over the meat.
Place the flour, egg and breadcrumbs in three separate dishes. Dust the meat first with flour, then dip in the egg, allowing the excess to drip back into the dish, then coat with the breadcrumbs. Chill for 30 minutes to set.
Fill a deep-fryer or large deep saucepan with oil and heat over medium-high heat until 170°C (a cube of bread will turn golden in 60 seconds). Working in batches, gently lower the crumbed meat into the oil and cook for 5 minutes or until the crumbs are golden and the meat is cooked through. Drain on paper towel and keep warm.
Using a sharp knife, slice each tonkatsu into 1.5cm strips, then transfer the strips to a plate.Serve with the mustard if using, sesame seeds, rice, tonkatsu sauce for dipping (although I like to pour some over the chicken to serve) and cabbage salad.