Cook the beans in lightly salted boiling water for 5 minutes. Add the parsley and coriander, cover and simmer for a further 5 minutes. Strain and reserve some of the cooking liquid.
Combine the beans with the chillies, garlic, cumin, 2 tablespoons oil and 3—4 tablespoons of the reserved cooking liquid in a food processor or blender. Process to a smooth paste and season to taste. Add a little more of the cooking liquid if the mixture is too dry. Transfer to a serving dish and chill.
Heat the rest of the oil in a nonstick pan and fry the onion briskly until golden and crisp. Spread over the dip. If liked, serve the dip with wholemeal pitta bread or crudités.