In a large bowl, toss the potatoes with the butter and olive oil, thyme, rosemary, salt and pepper. Arrange the potatoes in the bottom of a 9x13-inch pan (in a single layer). Roast, flipping once, until browned (about 30 minutes). Add the broth and garlic cloves to the pan. Roast until most of the broth is absorbed and the potatoes are very tender, about 15 minutes more. Serve hot with juices drizzled over.