Place the potatoes in a pot with enough water to cover.
Bring to a boil and cook until soft and easily pierced with a fork, about 20 minutes.
Meanwhile, in a separate pot, bring about 4 cups (960 ml) of water to a boil.
Add the edamame and cook for 5 to 7 minutes.
Drain the edamame well.
Transfer the cooked edamame to a blender or food processor and pulse a few times until roughly chopped. Be careful not to purée them; you want a coarse, chunky texture.
When the potatoes are done, drain them, reserving about 1/4 cup (960 ml) of the cooking water. Return the potatoes to the hot pot.
Add the chopped edamame and the miso-water mixture to the potatoes. Use a potato masher or a large wooden spoon to smash everything together until it reaches a chunky consistency.
If the mixture seems too dry, stir in a little of the reserved potato water until you reach your desired creaminess.
Taste and adjust the seasoning with salt, pepper, or more miso if needed.
Garnish generously with chopped scallion and serve warm.
