Wash and slice the tomatoes. Peel the cucumber and cut into slices or small cubes. Cut the onion into thin slices.
Put everything in a bowl. Sprinkle with a little salt and mix well. Cut some chives and some parsley into small pieces. Cut the peppermint into fine strips and add to the bowl with the other herbs.
In a separate bowl, mix the vinegar with salt, pepper and a pinch of sugar. Add about 1 tablespoon of water and then the oil. Pour the dressing over the salad and mix well.