Place the potatoes and garlic cloves in a large pot and cover with cold, well-salted water by 2 inches.

Bring to a boil, then reduce to a simmer and cook 15 to 18 minutes, until a knife slides into the largest piece with no resistance.

Drain well and return the potatoes and garlic to the warm pot for 1 minute to steam off excess moisture.

Pass the potatoes and garlic through a ricer back into the pot, or mash by hand until smooth.

Stir in the butter until melted, then fold in the warm milk and sour cream.

Season with salt and pepper, tasting as you go.

Keep warm, covered, over very low heat or in a slow cooker on low for up to 2 hours, stirring in a splash more warm milk if they stiffen.