1. Wash and cut the potatoes in half and place them in a baking dish with the onion. Drizzle oil and vinegar over the top and sprinkle with salt.
2. Bake the potatoes in the middle of the oven for about ½ hour at 225 C.
3. Let them cool slightly, and then add the cheese. Cover with lots of fresh watercress and serve.
As an alternative to watercress you can use fresh rucola salad. It also tastes great with some toasted pumpkin seeds sprinkled on top.