Heat 1 teaspoon of oil in a large frying pan over medium heat. Tip in the sweet potato chunks and fry for 4-5 minutes, until nicely golden on all sides. Remove from the pan with a slotted spoon and set aside.
Reduce the heat slightly and add the remaining oil. Once melted, add the spices. Fry for 1-2 minutes, until aromatic.
Reduce the heat again, add the onion and fry very gently, stirring from time to time, for 5-6 minutes, until softened. Stir in the garlic and green pepper and continue to fry for a further 2-3 minutes, until the pepper has taken on a little color and the garlic has softened.
Add the chopped tomatoes and purée, tip the sweet potato back into the pan, turn the heat up a little and simmer for 6-8 minutes, until the mixture has slightly reduced and thickened. Drain and rinse the kidney and black beans, add to the pan and simmer for a further 5 minutes, seasoning to taste. By now, the mixture should be fairly thick and the sweet potato tender. Serve with guacamole and salsa.