First Step: Prepare Potatoes and Aromatics Generously butter a 2 1/2-quart gratin or baking dish to prevent sticking. Peel and cut 3 pounds of russet potatoes into 3-inch chunks. Place them in a large pot and cover with cold water about 2 inches above the potatoes. Add 1 tablespoon kosher salt, 6 whole peeled garlic cloves, 1 bay leaf, and 4 thyme sprigs. Bring the pot to a boil over high heat.

Second Step: Simmer Potatoes to Tenderness Reduce heat to medium-low and partially cover. Simmer until potatoes are tender but still intact, about 20 minutes. Drain potatoes thoroughly and let them stand uncovered for about 3 minutes until edges look dry and chalky. Discard the bay leaf and thyme stems.

Third Step: Mash and Combine Ingredients For smooth texture, press hot potatoes and garlic through a food mill or ricer. Avoid electric processors to prevent gluey texture. Alternatively, mash gently with a hand masher until smooth. Stir in 8 ounces room-temperature cream cheese, 6 tablespoons softened unsalted butter, 1 cup full-fat sour cream, and 3/4 cup warm whole milk. Add additional milk as needed to reach your preferred creaminess. Season with 1 teaspoon seasoned salt and freshly ground black pepper to taste while the mixture is still warm.

Fourth Step: Assemble and Refrigerate Transfer the creamy mash into the prepared dish, smoothing the surface. Dot the top with chilled bits of butter for rich flavor and a beautiful finish. Cover tightly with plastic wrap and refrigerate overnight or up to 2 days to allow flavors to meld and save oven time on serving day.

Fifth Step: Bake to Golden Perfection Before baking, remove the casserole from the fridge about 2 hours ahead to approach room temperature. Preheat oven to 350°F (175°C). Remove plastic wrap and bake uncovered for approximately 1 hour until the top is golden brown and the center is heated through.

Final Step: Serve and Optional Enhancements Let the casserole cool slightly before serving. For extra richness, optionally top with shredded cheese and cooked bacon before baking. Bake covered with foil for 35 minutes then uncovered for 10 minutes until bubbly and golden. Garnish with chopped green onions or fresh herbs for color and flavor boost.