Prepare the potatoes as jacket potatoes. Let cool, peel and cut into slices. Clean the tomatoes and cut into pieces. Cut the feta into small cubes. Peel and finely chop the onions.
Heat the oil in a pan and fry the onions in it.
Mix the potatoes, tomatoes, feta and onions in a bowl.
Mix the mayonnaise with a little preserving liquid from a jar of pickled cucumbers; alternatively, you can stir in a little water and mild vinegar.
Stir the dressing into the salad. Let sit in the fridge for a few hours before serving.