Heat oven to 400°F (205°C). Slice just enough off the garlic bulb to expose the clove tops. Drizzle with olive oil, wrap in foil and place in a small baking dish. Roast 40–50 minutes until cloves are soft and golden; cool just enough to handle.

While garlic roasts, put peeled potato chunks in a large pot and cover with cold unsalted water by 1 inch. Bring to a gentle boil, then cook 15–20 minutes after boiling, until a fork slides through the centers easily.

Drain thoroughly. Return potatoes to the pot over low heat for 1–2 minutes, shaking gently, to evaporate surface water without browning. Remove from heat.

Squeeze roasted garlic from the skins and mash it with a fork; reserve a few small pieces for the top if desired. Mash potatoes with a hand masher or pass through a ricer. Fold in garlic and butter, then gradually add warm milk, salt and pepper. Stop when soft and creamy; do not blend or overwork.

Serve warm with any reserved measured garlic. Refrigerate leftovers promptly in shallow containers within 2 hours, or 1 hour above 90°F, at 40°F or below. Use within 3–4 days and reheat to 165°F, adding a little milk if needed.