Heat oven to 300°. Wrap garlic in foil, and roast until garlic is turning golden and heads are soft, about 1 hour. Let stand until cool enough to handle. Slice off tip of each head, just enough to expose cloves; press out garlic, using tip of knife to help. Discard garlic skin; transfer garlic to small bowl. Â Add 4 1/2 teaspoons olive oil, salt, and pepper. Make a paste using the back of a spoon.
Raise oven temperature to 475°. Slice each potato crosswise into one-inch-thick pieces. Spread a generous amount of paste on each slice; reassemble potato, pressing so the potatoes stick together. Set potato on a piece of foil, drizzle with 1 teaspoon olive oil, salt, and pepper to taste. Wrap potato tightly in foil; repeat process with the other potatoes. Bake potatoes in a roasting pan or on a baking sheet until they are soft and have a caramelized roasted-garlic crust on the bottom, about 1 hour and 10 minutes. Serve crust side up on a bed of fresh thyme.