Step 1

Preparation 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.

Step 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.

Step 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).

Step 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.

Step 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.

Step 6

Cooks' NoteWe love the silky texture that a ricer gives to these mashed potatoes, but if you don't have one or prefer a chunkier potato, feel free to use a hand masher instead.