Boil the 500 grams potatoes in ample water until soft enough to stick a fork through.
While the potatoes are cooking, put the sliced ¼ onion and 2 Japanese cucumbers in a bowl with the 1 teaspoon kosher saltand add some water. This pickles them, breaking down the cell walls of the veggies and releasing water making them more crunchy. Let them sit for about 10 minutes then squeeze out as much of the water as you can using your hands (the more water you get out, the crunchier they'll be).
Whisk the 1 tablespoon lemon juice, 1 tablespoon olive oil, 1 tablespoon mayonnaise, and ½ teaspoon kosher salt to make the dressing.
Drain the potatoes well and put into medium bowl and cover with the dressing while still warm. Gently combine the potatoes and dressing trying not to smash the potatoes too much. Add the onions, cucumber, and 1 tablespoon capers and mix until evenly distributed. Taste to make sure it's okay, you can add more lemon, more salt, or even a pinch of sugar if you want to mellow it out a bit.
Serve it warm or let it come to room temperature.