Put oven rack on the lowest level and pre-heat oven to 425° F. for at least 15 minutes.
Pour enough olive oil into a large cast-iron skillet, tilting it, just to cover the bottom of the pan. Strip the leaves from the rosemary sprig and scatter evenly over the bottom of the pan. Sprinkle the salt over the rosemary.
Scrub the potatoes, if necessary, then cut them in half, and set them cut side down on the rosemary and salt. Roast on the lowest oven rack until the potatoes are tender and the bottoms are crisp and well browned, about 30 to 40 minutes.
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