Sticky barbecue pork loin with apple chimichurri slaw

Name: Sticky barbecue pork loin with apple chimichurri slaw
Ingredients: 1 tsp mustard powder; 1 tsp smoked paprika; 1 tsp chopped rosemary; 2 tbsp dark brown sugar; 2 tbsp cold pressed rapeseed oil; 500g/1lb 2oz pork loin cut into 2.5cm/1in-thick fillets; salt and freshly ground pepper; ½ bunch flat leaf parsley; ½ bunch coriander; 8 mint leaves; 1 garlic clove, crushed; 2 green chillies; 5 tbsp olive oil; 1 tbsp sherry vinegar; 1 Granny Smith apple; 300g/10½oz celeriac; 2 tbsp Greek-style yoghurt; 1 tsp Dijon mustard; black garlic barbecue sauce; 4 brioche buns halved