Preheat oven to 425°F. Place potatoes, rosemary sprigs, smashed garlic, 1/4 cup of the salt, and water to cover in a 3-quart saucepan. Bring to a boil over high; reduce to medium, and simmer until potatoes are tender when pierced with a fork, 10 to 15 minutes. Drain; discard rosemary sprigs and smashed garlic.