Boil the potatoes, peel them and cut them into slices while they are still warm.
Cut the onion into small cubes and pour the hot broth over it. Then mix with mustard and vinegar and pour over the potatoes. Let it steep for about 2 hours.
Halve the (half) cucumber lengthways and scrape out the core with a small spoon. Cut into thin slices. Halve the spring onions and also cut into small pieces. Pour under the potatoes together with the oil and let it steep for another 1/2 hour. Season to taste with salt, pepper and parsley.