Peel and finely chop the onions. Peel and chop the sweet potatoes into small dice around 1cm across.
Warm a large pan over a medium heat for 1 min. Add the onions with the oil. Season with a pinch of salt and pepper. Cook for 5 mins, stirirng occasionally, till the onions have softened a little.
While the onion cooks, peel and crush the garlic. Halve the chilli. Flick out the seeds and white bits (or leave them in for more heat). Finely chop them. Halve the peppers. Scoop out the seeds and white bits. Roughly chop them. Drain the kidney beans and rinse them.
Stir the garlic and chilli into the onions. Add the cumin, ginger, cayenne and chilli. Give everything a good stir to mix it.
Add the peppers, potatoes, kidney beans and chopped tomatoes to the pan. Fill the tomato tin with water (around 400ml) and add that too. Pop on the lid. Bring to the boil, then turn the heat down and simmer for 30 mins, stirring now and then.
Take off the lid and cook for another 5-10 mins till the chilli is nice and thick. taste the chilli and add more salt and pepper if you think it need sit. Ladle into warm bowls and garnish with chopped herbs.