Preheat the oven to 400°F. Slice off the top of each head of garlic, between ¼ and ½ an inch, to expose the tops of every clove. If a clove is still covered in paper, surgically remove the top with a paring knife.* (see note)
Place the whole garlic heads on a cookie sheet, in a muffin tin, or a Dutch oven, cut side facing up. Drizzle with olive oil. If any of the cloves are bare, rub on some olive oil with your finger.
Rub any of the larger top pieces in olive oil that has dripped onto the bottom of the roasting dish. Tuck them into the roasting dish, exposed side facing up. These will roast faster so keep an eye on them to prevent them from burning.
If using a cookie sheet, cover each head with an overturned ramekin or oven-proof dish. If using a muffin tin, cover with an overturned cookie sheet. If using a Dutch oven or casserole dish, cover with the lid.
Roast the garlic for from 45 minutes to one hour, until caramelized and soft. Remove from the oven. When cool enough to handle, pop the cloves out of the head with your fingers. They will slide out easily.
Use the roasted garlic immediately or refrigerate. The garlic will keep in the refrigerator for at least 5 days.