How to Make It
Preheat the oven to 400°.
Place the potatoes in a large pot of salted water and bring to a boil. Cook, uncovered, until the potatoes are fork tender, about 15 minutes. Drain and return to the pot.
Add the butter and mash the potatoes until smooth. Stir in the milk and season with salt and pepper to taste.
Transfer the potatoes to a 3-quart ceramic baking dish and sprinkle the cheese over the top. Bake for 15 minutes, until the cheese is melted and beginning to brown. Garnish with the scallions and serve warm.