Heat the oil in a large frying pan and fry the onion, stirring, until it begins to colour. Add the tomatoes, ginger, saffron, chilli pepper and some salt. Cook for about 20 mins over a low to medium heat until the sauce is reduced.
Add the prawns and cook for 3-5 minutes until they turn pink, turning them over once. Stir in the coriander and parsley before serving immediately.