1. Place the potatoes in a large pot and fill with water to cover the potatoes by 2 inches.
2. Bring to a boil; then lower the heat to a gentle boil.
3. Cook until the potatoes are tender, about 30 minutes.
4. Test for doneness by gently inserting a paring knife through the middle.
5. If it goes in smoothly, your potatoes are done.
6. Drain under cool running water and transfer to a bowl.
7. Gently peel off the skins and discard.
8. Run the hot potatoes through a potato ricer over a large pot (the same pot you boiled the potatoes in is fine). (If you don't have a ricer, use a potato masher; the potatoes come out creamier if you use a ricer.)
9. Place the pot on the stove over medium heat and add 1/2 cup of the warm milk and half of the butter.
10. Mix well until the butter has melted. Add the other 1/2 cup of milk, the rest of the butter, the scallions (if you are making the potatoes in advance, don't put the scallions in at this point; wait until you are ready to serve), and the salt. Mix well, turning constantly. Taste and add more salt if needed.
11. Serve in a warmed bowl. Add 1 tablespoon of softened butter, if you wish, crack some fresh pepper on top, and sprinkle on lemon zest.