Finely chop the onion, carrot and celery. Cut the sweet potato into chunks and grate the garlic into a fine paste.
Heat the oil in a large, heavy based casserole. Fry the onion, carrot and celery for 10 minutes until softened. Add the sweet potato and garlic and cook for a minute more.
Add the spices and cook for a minute before adding the chipotle. Stir through the tomatoes along with a splash of water. Cover and simmer gently, for 30-40 minutes until the vegetables are tender.
Once the sweet potato is tender and the sauce thick and rich, add the lentils. Drain and rinse the beans and stir through and simmer until heated through.
To serve, add to bowls, top with a good dollop of soured cream, tortilla chips, cheddar and coriander leaves.