For the marinade, whizz the chillies, garlic, sugar, maple syrup, ginger, mustard, thyme, oil and seasoning until smooth. Coat the pork with the marinade. Cover and chill for 2 hours, or overnight, if possible. Remove from the fridge 30 minutes before cooking.
Preheat the oven to gas 7, 220°C, fan 200°C. Season the pork. Cook for 20 minutes, then pour in the stout around the meat, cover with foil, and reduce the temperature to gas 1, 140°C, fan 120°C. Roast for 5 hours, basting occasionally with the meat juices.
Remove the foil and turn up the heat to gas 7, 220°C, fan 200°C. Cook for 20 minutes; remove from the oven and leave to rest, covered, for 20 minutes.
Meanwhile, make a gravy. Pour the meat juices into a pan. Skim the fat from the surface, and then boil for 5 minutes, until thickened slightly. Transfer to a serving jug.
Using 2 forks, shred the pork onto a serving plate and drizzle over a little of the gravy. Serve with warm tortillas, the remaining gravy and some Sweet cabbage slaw, if you like.
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