For me, this is a kitchen staple. If you like, you can draw off part of the oil remaining in the baking dish after roasting the garlic to use separately as a flavored oil.
Preheat the oven to 375°F. Peel the outermost layers of skin off the heads of garlic. Cut off the top one-third of the heads to open the cloves. Save the small pieces of garlic for another use (see Chef's Note). Put the heads, cut sides up, in a small baking dish and pour the olive oil over them. Season with salt and pepper.
Cover tightly, place in the oven, and roast until about three-fourths cooked, about 45 minutes. Uncover and return to the oven until the cloves begin to pop out of their skins and brown, about 15 minutes. Let cool.
When cool enough to handle easily, squeeze the roasted garlic into a small bowl. Press against the skins very well to get out all the sweet roasted garlic you can. Add the oil from the baking dish and mix well until a paste forms. Store, tightly covered, in the refrigerator, for up to 1 week.
Chef's Note: It is hard to have too much roasted garlic around. You can roast the little bits from the tips of the garlic heads. Put them in a separate small baking container, such as an individual custard cup. Season with salt and pepper, douse with olive oil, cover, and place in the oven to bake along with the whole garlic heads. Depending on their size, they will be soft and browned in about half the time needed for the whole heads. The little pieces make a good "cook's snack" while preparing dinner, or can be squeezed into tomato sauce or onto pasta.
