Cut the onion into fine rings and place in cold water for an hour, then drain well. Dip the tomatoes (if you want) briefly in boiling water and then peel them. But it can also be done without peeling, if the tomatoes are nicely ripe. Cut into thin slices.
Put the tomatoes and onions in a salad bowl. Mix with salt, pepper, oil and lemon juice. Mix in the coriander leaves and sprinkle on top. Put in the fridge for 20 minutes and serve.
This salad is served everywhere in Chile and is therefore called Ensalada Chilena. Goes as a starter, but also goes particularly well with a grilled piece of meat in summer.