Maltese Olive Oil Gratin | TASTE

Peel potatoes and slice on a mandoline to 4.5mm or 3/16 of an inch, just a little thicker than the width of a nickel. Set aside and peel. Slice the onion in half, and then slice each half on the mandoline on a slightly thinner setting. It's okay if the onion slices don't maintain their shape. Layer the onion and potato, starting with a layer of onion, in a 2-quart ovenproof dish, sprinkling each layer with salt, pepper, and rosemary, if using, followed by a generous coating of olive oil. Overlap the slices of potato on each other by a third so that they are evenly shingled. When you get to the final layer of potato, press down on the gratin to tightly fit everything into the dish, and gently add ¾ cup of boiling water. Bake uncovered at 400°F for an hour, or until the potatoes are completely tender in the center and the top has started to brown. Turn the heat up to 500°F for another 5-10 minutes until the gratin has crisped up to your liking. Let cool for at least 10 minutes before serving-it will firm as it rests.