The salt in this dish absorbs moisture as the potatoes cook, giving them soft, velvety flesh and crackled skin. Serve them with creme fraiche or sour cream.
Heat oven to 450 degrees. Rub the garlic over the bottom and sides of a 12-inch-round gratin dish or a cast-iron skillet. Brush a thin layer of olive oil over the pan. Sprinkle the rosemary over the bottom of pan, and place potatoes in a snug layer on top.
In a large bowl, whisk together the salt and flour. Slowly stir in 1/2 cup water until thoroughly combined. Pour the salt mixture over the potatoes in an even layer, making sure to fill in any gaps between the potatoes.
Roast potatoes 1 hour 15 minutes. Remove pan from oven, and let potatoes cool 5 minutes. Invert pan onto a serving platter or into a large bowl. Drizzle potatoes with olive oil, and serve with sour cream.