Clean the potatoes. Then dice the potatoes, leaving the skins on. Place them in a large pot and cover with 1 inch of water. Bring to a boil and boil until tender, about 15 to 20 minutes (pierce a piece of potato with a fork to assess doneness). Then carefully drain them.
When the potatoes are done, melt the butter in the same pot. Add the garlic and saute for 1 to 2 minutes, until fragrant but not browned. Add the potatoes back to the pot and pour in the milk.
Mash the potatoes with a masher, adding more milk if desired to get to a thinner consistency. Add the Parmesan cheese and season with the kosher salt and lots of fresh ground pepper. Taste and adjust seasonings as desired, adding more cheese, salt, or pepper as necessary.
If desired, thinly slice the green onion and sprinkle on top as a garnish. Serve immediately.
Notes
*For vegan substitutes, use olive oil and almond milk or your favorite plant milk.