Peel and cut potatoes into bite-sized pieces. Soak in water for 10 minutes and drain. Slice green onion thinly after cutting it lengthwise.

Defrost edamame and remove beans from pods.

Boil potatoes in a pot with enough water to cover them. Cook over medium heat until boiling, then cover and simmer for 10 minutes until tender. Drain water.

Return potatoes to the pot and heat over medium while shaking the pot to make them fluffy. Mash potatoes in a bowl to desired consistency, then mix in vinegar, salt, and pepper. Let cool.