Cut potatoes into bite-sized pieces and soak in water for 5 minutes. Drain.

Mince the green onion.

Dice cucumber into 1 cm cubes, sprinkle with salt, let sit for 5 minutes, rinse, and drain. Drain the tuna can.

Boil potatoes in a pot with enough water to cover them. Cook for 10 minutes until tender, then drain.

Mash potatoes in the pot, add vinegar and salt, and mix. Cook over medium heat to evaporate excess moisture.