Heat the oil in a saucepan, add the onion, garlic, chilli powder, coriander and cumin and cook over a medium heat, stirring frequently, for 5 minutes until the onion is softened. Add the beans, tomatoes and stock and season with salt and pepper. Bring to the boil, then reduce the heat, cover and simmer for 15 minutes.
Transfer to a food processor or blender and process until fairly smooth. Pour into flameproof bowls.
Scatter the tortilla chips on top of the soup and sprinkle over the Cheddar. Cook under a preheated hot grill for 1–2 minutes until the cheese has melted. Serve immediately with soured cream.