Preheat the oven to 200°C.
Mix pork mince, breadcrumbs, grated red onion, chopped parsley, mustard, soya sauce and grated apple. Shape into 4 burger patties and chill for 10 minutes.
To make the caramelised onions: Heat olive oil and butter in a large frying pan. Saute the red onions, sliced into wedges, until translucent. Add treacle sugar and fry for 5 to 10 minutes, stirring regularly, until caramelised and sticky.
To make the basting sauce: Boil the cayenne pepper, chilli powder, lemon juice, honey, tomato sauce, white wine vinegar and soya sauce for 5 minutes, then set aside.
Heat some olive oil in a large frying pan and brown the burger patties on both sides, basting regularly with the sauce. Transfer to a baking tray, brush with the remaining sauce and roast for 7 minutes.
Spread 4 toasted bread rolls with mayonnaise, add fresh rocket leaves, a pork burger and a spoonful of caramelised onions.