Position rack in lower third of oven; preheat to 450°F.
Peel and discard papery outer layers of garlic head. Cut about ¼ inch from the top of the cloves. Place the garlic on a piece of foil big enough to enclose the head. Drizzle with 1 teaspoon oil; seal the foil securely. Place on a large rimmed baking sheet.
Toss potatoes in a large bowl with the remaining 1 tablespoon oil, salt and pepper. Spread evenly on the baking sheet. Roast, stirring once or twice, until the potatoes are tender and browned, 20 to 25 minutes.
Remove the garlic from the foil; let rest until cool enough to handle. Combine melted butter and thyme in a small bowl. Squeeze the roasted garlic cloves out of their skins (discard skins). Add the roasted garlic to the butter mixture; mash with a fork (it'll be lumpy). Pour the garlic butter over the potatoes. Season with pepper to taste. Toss to coat.
