Heat oven to 450°. 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 ½ 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 in to a large a large bowl. Drizzle potatoes with olive oil, and serve with sour cream.