How to Make It
Place potatoes, onion, and cold water to cover in a large pot. Bring to a boil over high; reduce heat to medium, and gently boil until potatoes are fork-tender, about 20 minutes. Drain potatoes and onion, reserving ½ cup cooking water. Discard onion. Return potatoes to pot, and coarsely mash using a potato masher.
While potatoes cook, melt butter in a small skillet over medium-high. Add 5 of the sliced scallions, and cook, stirring constantly, until softened, about 2 minutes. Remove from heat, and stir in miso. Add miso-scallion mixture, salt, and several grinds black pepper to potatoes. Slowly stir in up to ½ cup reserved cooking water, about 2 tablespoons at a time, until potatoes are creamy. Serve potatoes topped with remaining scallion slices.