First, peel and finely dice the onion. Clean the cucumber, scrape out the seeds, and cut it into thin slices.
Clean the potatoes under running water then transfer them to a saucepan. Add salt and enough water to cover the potatoes, then boil them for 20-25 minutes (see notes).
While the potatoes are cooking, arrange the sliced cucumber on a plate, sprinkle some salt on top, cover it, and allow it to release some of its water.
Allow the potatoes to cook until done, drain the water and allow them to sit for 5-10 minutes. Next, peel the potatoes while still warm and then cut them into slices, each about ½ - 1 cm thick (approximately 0.2 - 0.4 inches). Transfer the potatoes to a bowl.
Prepare the vinaigrette: heat a tablespoon of oil in a saucepan under low to medium heat. Add the onion, fry until translucent, add half of the broth, combine, and allow to cook for about 5 minutes, remove from the pan, then set aside. To the saucepan, add the vinegar, apple juice, mustard, sugar, salt, and pepper, then combine. (see notes).
Drain excess water from the cucumber, then add this to the bowl with the potatoes. Pour the dressing on top of the potatoes while they are still warm, top with the cooking oil, then combine all the ingredients.
Add the remaining broth, a little at a time, then mix.
Garnish the salad with chopped chives or as preferred, serve it warm, or allow it to chill in the fridge for at least 30 minutes, before serving.