Put the peeled and chopped potatoes in a pot and cover with water. Bring the water to a boil over high heat then turn down to a simmer. Boil the potatoes until fork-tender, about 15-20 minutes. Drain the potatoes well and mash with the butter and miso paste. Stir in the milk and green onions, taste and season with salt and pepper, adding more butter and milk if needed. If desired, pop into a 400°F oven to toast the top until lightly brown, around 20 minutes. Enjoy hot.