Boil the potatoes in salted water until tender - about 12 minutes depending on size.
Halve the cucumber lengthways, scrape out the seeds and cut the cucumber into slices.
Drain the potatoes and spread them out on a baking tray lined with baking parchment.
Spray the tops with a little oil spray and smash the potatoes with the base of a glass.If they stick when you lift the glass, rub the base with a little oil and give the glass a twist once the potato is smashed. That way they release more easily without pulling half the potato with them.
Season the potatoes with salt and freshly ground pepper and bake them in the middle of a preheated conventional oven (top and bottom heat) at 200°C for about 35 minutes.
Let the potatoes cool, then chop them into rough chunks with a knife.
Mix the ingredients for the dressing and season to taste with salt and freshly ground pepper. Bear in mind that the potatoes are already seasoned.
Put the potatoes, cucumber and red onion in a bowl, pour over the dressing and mix well.