Cook the beans in a pan of boiling salted water for 5 minutes. Add the herbs, cover and simmer for a further 5 minutes. Drain, reserving some of the cooking liquid.
Place the cooked beans in a blender or food processor with the chillies, garlic, cumin, 2 tablespoons of the oil and 3–4 tablespoons of the reserved cooking liquid. Process to a smooth paste, season to taste and add a little more cooking liquid if it is too dry. Transfer to a serving dish and chill until required.
When ready to serve, heat the remaining oil in a non-stick frying pan and fry the onion briskly until golden and crisp. Spread over the dip and serve with crudités or wholemeal pitta breads.