Ingredients

• 4 pounds Yukon Gold potatoes, peeled, cut into 2" pieces
• 6 large garlic cloves, peeled
• 1 tablespoon plus 2 teaspoons kosher salt, divided, plus more
• 1 1/4 cups whole milk
• 4 thyme sprigs
• 3/4 cup (1 1/2 sticks) plus 2 tablespoons unsalted butter, divided
• 3/4 teaspoon freshly ground black pepper, plus more
• 1/2 cup sour cream
• A potato ricer or food mill

Instructions

1. Cover potatoes, garlic, and 1 Tbsp. salt with cold water in a large pot. Bring to a low boil, then reduce heat and simmer (do not boil) until potatoes are very tender when pierced with the tip of a paring knife but not falling apart, 20–25 minutes.
2. Meanwhile, heat milk, thyme, and 3/4 cup butter in a small pot over medium, stirring, until butter is melted. Remove from heat; set aside.
3. Drain potatoes and garlic; return to pot. Toss over low heat until moisture evaporates, 1–2 minutes. Using potato ricer or food mill, immediately press potatoes and garlic into a large bowl (do not let cool).
4. Discard thyme from milk mixture and gradually stir into potatoes, reserving about 1/2 cup if you plan to make in advance (see Do Ahead). Season with 2 tsp. salt and 3/4 tsp. pepper. Fold in sour cream and stir with a spoon until incorporated and very smooth (do not overmix or potatoes will become gummy). Taste and adjust seasonings. Serve topped with remaining 2 Tbsp. butter and more pepper.
5. Do Ahead Mashed potatoes can be made 2 hours ahead; cover and store at room temperature. Or chill, covered, up to 8 hours; reheat over medium with reserved 1/2 cup milk mixture.
6. Mashed potatoes can be made 2 hours ahead; cover and store at room temperature. Or chill, covered, up to 8 hours; reheat over medium with reserved 1/2 cup milk mixture.