Finely chop the onion and sauté in a pan with a little oil until soft. Remove from the heat and allow to cool.
Next, in a large bowl soak the panko in the milk, add the minced meat and mix together by hand. Then add the cooled cooked onions, egg, worcestershire sauce and soy sauce, as well as salt and pepper to taste. Mix well once more.
Separate the mixture into 4 equally sized portions and mould into rounded patties, indenting the centre a little with your finger to ensure it cooks through evenly. Dust each patty with plain flour and dip into the beaten egg.
Spread the panko breadcrumbs on a plate and cover the patties on both sides with the breadcrumbs. Fill a wok with oil for deep frying. Heat the oil until a small sprinkle of panko sizzles when added.
Deep fry the burger patties in cooking oil on a medium-high temperature (about 150°C/300°F) turning several times until golden brown on the outside. Remove from the oil and drain on a wire rack.
To serve, make a Menchi Katsu Burger by slicing and buttering a brioche bun, adding a handful of shredded white cabbage, a menchikatsu burger and drizzling with tonkatsu sauce. Alternatively make a Menchi Katsu Sandwich with white bread and cabbage, or simply serve on a plate with rice, shredded cabbage and tonkatsu dipping sauce.
• Menchi Katsu are very versatile and are fantastic the next day in a bento lunchbox.
• If you prefer a less oily cooking method, drizzle with oil and oven cook for 30-40 minutes at 450°F/230°C/Gas Mark 8.