1. For the apple katsu sauce,heat the butter in a small saucepan over medium heat and fry the onions until softened. Add the apples,sugar,lemon juice and vinegar and simmer for about 10 minutes until the apples are softened. Add the tomato sauce,worcestershire sauce,soy sauce and mustard and stir to combine. Puree with a stick blender to a smooth sauce,adjust for seasoning,and set aside.
2. For the katsu,combine a half cup of the panko crumbs with the stock or milk and set aside to soften for five minutes. With a cleaver or heavy knife,roughly mince the pork belly and combine with the pork mince,soaked panko mixture,onion and salt and mix to combine. Form into four oval-shaped patties and refrigerate for 10 minutes to firm.
3. Put the egg,flour and remaining panko into separate shallow bowls for dredging. Coat each patty in egg,then flour,then egg again,and finally panko crumbs. Refrigerate for a further 10 minutes to firm.
4. Heat the oil in a large saucepan to 175C. Deep-fry the katsus for around eight minutes until golden brown and cooked through. Drain on a wire rack and serve with the shredded cabbage and apple katsu sauce.
If you love this recipe,check out Adam Liaw's Persian pan kebabs with saffron rice.