Preheat the oven to 400 degrees F.
Slice off the top each head of garlic to expose some of the cloves inside. Place the head on a piece of foil. Drizzle with olive oil and wrap in the foil. Roast until cloves are lightly browned and tender, about 30 minutes.
Cut the peeled potatoes into pieces and cook in boiling water until fork-tender. Drain water and return to pot, and roasted garlic, milk and mash until smooth.
Add salt to taste.