Dice the sweet potatoes into 1cm (⅓in) cubes and cover with cold water, then boil until just cooked (be careful not to overcook as they will break up). Drain carefully, keeping the liquid.
Drain the beans and put them into the pot you have just used for the sweet potatoes. Add the potato cooking liquor, adding enough hot water to cover the beans by a few centimetres, and bring to the boil. Skim off any foam that comes to the surface and cook on a gentle boil for at least 45 minutes. Top up with more hot water if it reduces too much. Test the beans to see if they’re cooked and continue until they’re done, when they should be drained.
Heat up the oils and fry the onion and cumin until the onion softens, then add the garlic and fry for a further 2 minutes. Add the lime juice and bring to the boil, then remove from the heat. Gently mix the sweet potato, beans, onion mix, coriander and 1 teaspoon salt together and leave to sit for at least an hour before using.
© 1997 Peter Gordon. All rights reserved.