Wash the tomatoes, pat dry and then cut into eighths or slices. Cut out the green stem. Put in a bowl, add a little salt and let it steep.
In the meantime, cut the onion into small pieces and mix the oil, vinegar, sugar, pepper and chives (and possibly marjoram, lovage, basil and, if you like, some water) into a marinade.
Pour over the tomato pieces and mix gently.
Variations: Add thinly sliced pickles or use parsley instead of chives.