The roasted garlic caramelizes to form a golden crust on the potatoes.
Heat oven to 425 degrees. Wrap garlic in foil, or place in garlic roaster, and roast until garlic is turning golden and head is soft, about 1 hour. Let stand until cool enough to handle. Slice off tip of head, just enough to expose cloves; press out garlic, using back of knife to help. Discard garlic skin; transfer garlic to small bowl. Add 1 1/2 tablespoons oil, 1 teaspoon salt, and 1/4 teaspoon pepper. Smash together with back of teaspoon to create thick paste.
Raise oven to 475 degrees. Slice each potato into three or four one-inch-thick pieces. Spread a generous amount of paste between each slice; reassemble potato, pressing on chunks so potato sticks together. Set potato on a piece of foil, drizzle with 2 teaspoons oil, and salt and pepper to taste. Wrap potato tightly in foil; repeat process with other potatoes. Bake potatoes on baking sheet until they are soft and have a caramelized roasted-garlic crust on the bottom, about 1 hour and 10 minutes. Serve on bed of fresh thyme.