Preheat the oven to 325°F. Line a baking sheet with parchment paper or aluminum foil.
Cut the top quarter off each garlic head so the cloves are exposed. Place cut side up on the prepared baking sheet. Drizzle the oil over the garlic and season lightly with the salt and pepper. Turn the garlic cut side down. Roast until the cloves are soft and golden brown, 1 to 1 1/4 hours.
Let the garlic cool. Squeeze the soft garlic flesh from each head into a bowl. Mash the flesh with a fork to blend it into a purée. (The purée can be stored in an airtight container in the refrigerator for up to two weeks.)
Yield: 1 cup