Heat the oil in a deep-sided frying pan and cook the onion for 8-10 mins until it starts to turn golden. Add the ginger, garlic and chilli and cook for 1-2 mins. Stir in the sugar and spices for 1 min, then splash in the vinegar and tomatoes. Season with salt and simmer for 5 mins, stirring, until the sauce thickens.
Stir in the prawns, reduce the heat and cook for 8-10 mins until cooked through – if the sauce gets really thick, add a splash of water. Remove from the heat, stir though most of the coriander. Serve straight from the dish scattered with the remaining coriander and the rice, yogurt, chutney and salad in separate bowls.