It is a myth that mashed potatoes need to be made just before they're served. They can go into a heat-proof bowl set over simmering water for about thirty minutes, as long as you keep adding more liquid to keep them creamy. Or, even easier, I pour them into a baking dish, sprinkle with Parmesan cheese, and bake them in the oven.
MAKE IT AHEAD: Assemble the dish, including the Parmesan, and refrigerate for up to 3 days. Bake before serving.
Preheat the oven to 375°.
Place the potatoes, garlic, and 1 tablespoon of salt in a large pot with enough water to cover the potatoes. Bring to a boil over high heat, reduce to a simmer, and cook for 20 to 25 minutes, until very tender.
Drain the potatoes and garlic and process them together through a food mill fitted with the coarsest blade set over a bowl. While the potatoes are still hot, stir in the goat cheese, butter, sour cream, half-and-half, 4 teaspoons salt, and 2 teaspoons pepper, until smooth.
Pour the mixture into a 9 × 12 × 2-inch oval baking dish, smoothing the top. Sprinkle the Parmesan on top and bake for 30 to 40 minutes, until lightly browned. Serve hot.