In a small bowl, stir together the miso and mirin.
Cut the potatoes into bite-sized pcs, soak them in water, then drain.
Cook the potatoes with salt in a pot - bring it to a boil and cook 6-8 mins.
Drain the potatoes, return it on low heat, gently shake the pot to help evaporate the moisture.
Add the miso mixture and mayo and stir
Garnish with green onions if desired
