Peel and chop the sweet potatoes into chunks around 2cm across. Trim the dry ends off the celery and finely slice it. Peel and grate or crush the garlic. Halve the chillies and scrape out the seeds and white pith. Finely chop them (if you like things hot – or not – see our spicing tip at the end).
Crumble the stock cube into a heatproof jug. Add 1 ltr boiling water and stir to dissolve. Drain the kidney beans into a colander and rinse under cold water.
Warm a large pan over a medium heat for 2 mins. Add 2 tbsp oil, the garlic, chillies and 2 tsp each ground cumin, coriander and smoked paprika. Cook and stir for 1 min. Add the sweet potatoes, celery and kidney beans. Pour in the vegetable stock. Add the chopped tomatoes.
Season with a pinch of salt and pepper. Give it a good stir, cover and bring to the boil. Turn the heat down and simmer for 20 mins, stirring now and then, till the sweet potatoes are tender when pressed with a fork.
Taste the hot pot and add a little more salt and pepper if you think it needs it. Ladle into warm bowls and serve straight away.