Scrub potatoes and place in large pot covered with water, bring to a boil. Boil gently until potatoes are pierced easily with a knife, about 30 minutes.
Drain well, you do not want any water left in the pot.
Mash potatoes with hand masher. Add butter and continue mashing until they are relatively smooth.
Add sour cream and stir to combine. Season with kosher salt and fresh ground pepper to taste. If the potatoes seem to dry you can add a splash of milk.
Add green onion, give a final stir to combine and serve warm.
We usually have leftovers, and these reheat well. Put them in a saucepan with a little milk and stir frequently.