Wash the potatoes well and cook them for about 20 minutes. Quench cold, peel and let cool off. Wash watercress, drain well, pluck leaves from the stems. Cut tomatoes into quarters.
Peel and chop the onion. Cut the potatoes into slices. Mix vinegar, salt and pepper and add oil. Mix with onions and potatoes. Leave to stand for about 30 minutes. Add cress and tomatoes.
Season to taste and arrange with the ham slices.