Wash and cut the potatoes in half and place them in a baking dish with onion. Sprinkle them with oil, vinegar and salt. Bake the potatoes in the middle of the oven for about ½ hour at 225 C. Let them cool slightly, and then add the cheese. Cover with lots of fresh watercress and serve.
Try also adding 100 g. fresh rucola salad and 1 handful of roasted pumpkin seeds to the potatoes. Serve it alone or as accompaniment for salmon or beef.