Heat the oil in a large frying pan or shallow casserole dish. Add the garlic, onion and chilli, cook for a few minutes, then add the prawns and cook them for about 4 minutes, by which time they should be pink all over.
When the prawns are cooked, add the tomato and cook for a minute or two. Season with salt and stir the herbs through.
Take the pan to the table, remembering to put it on a heatproof mat. Eat with bread or rice to mop up the juices.