Other than tasting good, my second favorite thing about these potatoes is that they can be made ahead of time, within 24 hours of serving and then baked off in the oven for 40 minutes right before service, easy.
2 tablespoons unsalted butter, melted, divided, plus 1/4 cup cut into 1″ cubes, room temperature
2 1/2 pounds potatoes, peeled, cut into 2″ pieces
4 ounces cream cheese, cut into 1″ cubes, room temperature
2 tablespoons prepared horseradish
2/3 cup whole milk, warmed
Freshly ground black pepper
1 bunch of green onions, white and pale-green parts only, minced (about 2/3 cup loosely packed)
1. Brush an 8x8x2″ or other 6-cup baking dish with 1 tablespoon melted butter. If baking off right away, preheat oven to 375°F.
2. Place potatoes in a large pot and add cold water to cover by 1″. Add a large pinch of salt; bring to a boil, then reduce heat to medium-low. Cover with lid slightly ajar and gently simmer until potatoes are tender, about 20 minutes.
3. Drain potatoes; return to same pot. Shake and stir with a wooden spoon over very low heat until dry. Then, using a potato masher, mash coarsely. Using a hand mixer, beat 1/4 cup butter into potatoes, a few pieces at a time, until blended. Beat in cream cheese, adding a few pieces at a time, then horseradish. With motor running, gradually add milk, beating until potatoes are light and fluffy, 4-5 minutes.
4. Season well with salt and pepper.
Stir in green onions. Scrape potatoes into prepared dish. Use a spatula to create peaks across the surface. Drizzle potatoes with remaining 1 tablespoon melted butter and sprinkle with paprika.
DO AHEAD: Potatoes can be made 1 day ahead. Let stand at room temperature to cool. Cover and chill.
5. Bake potatoes, uncovered, until they are heated through and top is golden, about 40 minutes (if chilled, add 10 minutes).