Boil the potatoes and let them cool until lukewarm. Then peel and cut into bite-sized pieces or slices. Peel the cucumber, remove the core if necessary and cut into very thin slices.
Bring the vegetable stock to the boil briefly, add the vinegar and mustard to the hot stock and stir until smooth with the whisk.
Mix the potatoes, cucumber and sauce and let stand for about 10-20 minutes.
This low-calorie grilled salad has 7 points per serving.