chopped fresh herbssuch as thyme or chives (optional, for serving)
Cut the potatoes into 1 1/2-inch chunks, keeping the chunks similar in size so they cook evenly. Place in a saucepan large enough to hold the potatoes and liquid to boil them. Add the salt and garlic. Cover with cool water, then bring to a boil over medium-high heat. Reduce the heat medium and let simmer gently, adjusting the heat as needed, until the potatoes are fork tender, about 15 minutes. Drain into a colander, shake off the excess water, and return to the pot.
Cut the butter into a few pieces, then scatter it over the potatoes. With a potato masher or wooden spoon, roughly mash together a few times (they don’t need to be smoothly combined at this point).
In a small saucepan or in the microwave, heat the milk until it is very warm to the touch but not boiling. Pour over the potatoes. Add the sour cream.
Continue to mash and stir the potatoes, just until the ingredients are evenly combined. The potatoes should be very creamy, but still have small pieces of potato remaining. Let stand for 1 to 2 minutes to thicken if needed. Taste and adjust the seasoning as you like (I usually add another pinch of salt). Enjoy hot (you can also transfer to a heatproof bowl, then keep the potatoes warm over a saucepan of simmering water). Serve hot, sprinkled with herbs and a few grinds of black pepper if you like.
TO STORE: Refrigerate in an airtight storage container for up to 3 days before serving.
TO REHEAT: For smaller servings, you can reheat them in a saucepan with a few splashes of milk to keep them from drying out. To make them ahead and reheat, place them in a large, heatproof bowl set over a pan of simmering water. Gently heat and stir and be careful to not over mash. Add splashes of warm milk (or broth) as needed while stirring if they seem dry. You can also reheat them in a baking dish in the oven—add a few pats of butter on top and cover with foil.
TO FREEZE: I do not recommend freezing mashed potatoes, as this could negatively impact texture, making them mealy.