Turn your slow cooker on to low. Heat a frying pan over a medium heat. Add the ancho and chipotle chillies and dry fry, stirring until fragrant, without letting them burn. Transfer to a small jug and cover with water, then leave to soften for 20 minutes.
Transfer the chillies and water to a mini food processor or the beaker of a stick blender. Blend until completely smooth. Tip into a small bowl and set aside.
In a large frying pan, heat 2 tsp of the oil over a medium heat and fry the onion, celery and garlic for 5-7 minutes, until softened but not coloured. Add the dried oregano, cumin, coriander, paprika and cinnamon stick and fry for a further 30 seconds. Add the chilli purée and cook for a further 1 minute.
Add the black beans with their liquid, the chopped tomatoes, sweet potato, quinoa, stock, Marmite and chilli mixture to the slow cooker. Stir well, replace the lid and leave to cook for 4 hours.
Meanwhile, tip the cashew nuts into a small jug and cover with water. Leave to soak for 3 hours, or until softened. Drain and rinse. Blitz until smooth in a mini food processor with 75ml of water and 2 tbsp of lime juice. Season, then briefly blitz again. Scrape into a small bowl, cover and leave to chill for 30 minutes.
Preheat the oven to 200°C/gas mark 6. Brush the tortillas with the remaining oil and cut into triangles with scissors. Arrange in a single layer on a baking sheet and bake for 7-8 minutes. Set aside.
When the chilli is done, season to taste, then serve in shallow bowls. Dollop on the cashew sour cream, then add the tortilla chips, crushing some roughly with your hands. Serve with chilli slices, coriander sprigs and lime wedges on the side.