Put the oil in a heavy bottomed saucepan and when hot add the onions and tomatoes. Cook over a high heat for 5 minutes, stirring.
Turn the heat down and stir in the peanut butter and 300 ml of the water, mixing together well. Simmer for 8-10 minutes, stirring occassionally to prevent it burning.
Add the rest of the water, the thyme sprig, chilli pepper and sea salt then stir in the chopped spinach. Cook over a moderate heat until the sauce has thickened, about 20 minutes.
Add the prawns and the smoked fish if using. Cook, stirring , for another 10 minutes.