Preheat oven to 400°F. Place Yukon Gold potatoes on a rimmed baking sheet or in a large baking dish and cover tightly with foil. Roast until very tender, 60-65 minutes.
Using side of knife, bruise or smash the garlic cloves. Using a vegetable peeler, remove a wide 3 inch strip of zest from lemon and add to pot. Save lemon for other use.
Place milk in a medium saucepan with garlic, thyme, peppercorns, butter and kosher salt.
Bring mixture to a simmer over medium high heat, stirring so bottom doesn't scorch. Remove from heat, cover, and let sit for 30 minutes.
When potatoes are done, use a small knife to remove skins, or leave skins on if you like (I like the roasted flavors they add). Mash them in a large pot with a potato masher, or pass potatoes through a ricer or food mill fitted with the fine disk into the pot.(See Note 2).
Pour infused milk through a mesh sieve into pot with potatoes, discard solids. Place potato mixture over medium heat and stir until milk is incorporated into potatoes. Taste and season with more salt if needed. Drizzle more melted butter on top before serving. Drizzle more melted butter on top before serving or cover and keep warm until ready to serve.