Place potatoes on a foil-lined rimmed baking sheet. In a small bowl, whisk oil, sage and garlic; season. Drizzle half the sage oil over the potatoes. Roast at 425 degrees , 30 minutes; brush with remaining sage oil. Roast until tender, about 30 minutes more; season. In a bowl, mix prosciutto and Fontina; sprinkle over the potatoes. Broil until browned, 3 to 5 minutes.