Cut a small cross in the base of each tomato and cover the tomatoes with boiling water in a heatproof bowl. Stand for 30 seconds and remove with a slotted spoon to a bowl of cold water. When the tomatoes are cool enough to handle, peel them with a small, sharp knife and discard the skin. Roughly chop the flesh.
Melt 40 g of the butter in a medium frying pan. Add the onion and garlic and cook over medium heat until the onion is light golden, about 10 minutes. Add the curry powder, flour and crumbled stock cube and cook, stirring, for 2 minutes. Add the tomato and water. Bring the sauce to the boil, reduce the heat to low, cover the frying pan with a lid and simmer for 8 minutes, or until the sauce thickens slightly.
Meanwhile, peel and devein the prawns, leaving the tails intact. Melt the remaining butter in a small frying pan over medium-high heat. Add the prawns and cook until they turn pink. Spoon the prawns and butter mixture into the curry sauce stir for a further minute, or until the prawns are well coated with the sauce. Serve with boiled rice.
To make a chicken or fish curry, simply cut the meat into bite-size chunks and cook as described below for the prawns, allowing an extra minute or two of cooking time for chicken.