Make Ahead Mashed Potato Casserole

​from the Kitchn

• 3 pounds small russet potatoes, peeled and cut into 3-inch chunks
• 1 tablespoon kosher salt
• 6 whole garlic cloves, peeled
• 1 whole bay leaf
• 4 (3-inch) thyme sprigs
• 1 (8-ounce) package cream cheese, at room temperature
• 6 tablespoons butter, at room temperature
• 1 cup sour cream (full-fat)
• 3/4 to 1 cup whole milk, warmed
• 1 teaspoon seasoned salt, or to taste
• Freshly ground black pepper, to taste

1. Generously butter a shallow 2 1/2-quart gratin dish or baking dish.
2. Place the potatoes in a large pot and cover with cold water to a depth of 2 inches. Add the kosher salt, garlic, bay leaf, and thyme. Bring to a boil over high heat, then reduce the heat, partially cover, and simmer until the potatoes are tender when pierced with a knife, about 20 minutes. Do not let the potatoes break apart or become waterlogged. Drain well and let stand until the potatoes steam dry and their edges look chalky, about 3 minutes. Discard the bay leaf and thyme stems.
3. Press the hot potatoes and garlic through a food mill or ricer into a large bowl. Alternatively, mash them as smooth as possible with a hand-held potato masher.
4. Add the cream cheese, butter, sour cream, and 3/4 cup of the milk to the warm potatoes; stir until smooth. The mixture will firm up as it chills overnight, so at this point it should be slightly softer than you want to serve it. Season with seasoned salt and pepper.
5. Scrape the potatoes into the prepared dish and smooth the top. Dot the top with the bits of chilled butter. Cover the dish tightly with plastic wrap and refrigerate overnight or up to 2 days ahead.
6. When ready to bake, preheat the oven to 350°F. Remove the plastic wrap and bake until the top is golden brown and the potatoes are heated through, about 1 hour. Serve warm.