When raw garlic cloves are crushed or chopped, a series of chemical reactions produces their familiar assertive aroma and flavor. By cooking garlic cloves whole, either peeled or unpeeled, these reactions are bypassed, and the resulting flavor is surprisingly mild.
The easiest and most flavorful method for preparing cooked garlic purée is to wrap four or five heads together in aluminum foil and roast them for about 1 hour, until they are soft. The pulp is then extracted with a food mill or drum sieve. If a paler purée is needed for finishing cream sauces, unpeeled garlic cloves can be cooked in simmering water until they are soft. Some recipes suggest peeling the cloves and simmering them in several changes of water to attenuate their flavor. This usually is not necessary because whole cooked cloves are quite mild.