Trim the pork of any sinew and place in a non-metallic dish. Toast the cashews in a dry frying pan until golden. Roughly chop and set aside.
To make the marinade, mix all the remaining ingredients together except the peanut butter. Spoon 2 tbsp of the mixture over the pork fillet, cover and leave to marinate in the fridge for at least 2 hours or overnight. Stir the peanut butter into the remaining marinade mixture. This will become the dressing for the accompanying peanut ’slaw. Keep covered and chilled until ready to use.
To make the ’slaw, scrape the watery seeds out of the cucumber and slice the flesh into matchsticks. Toss the cucumber, shredded cabbage, carrot, spring onion and chopped coriander with the reserved peanut dressing in a bowl, until evenly coated. Scatter over the reserved toasted cashews.
Barbecue the pork for 15 minutes. Use barbecue tongs to turn the fillet often, until it is cooked through and evenly charred. Alternatively, cook in a griddle pan set over a high heat. Cover with kitchen paper and leave to rest for a few minutes before cutting into thick slices. Serve the pork warm, along with the peanut ’slaw.