In a medium pot, pour enough oil so that the oil comes 3 inches up the sides. Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 350 degrees F. (If you don't have a thermometer, a cube of bread will brown in about 3 minutes.)
Slice the potatoes thinly on a mandoline. As you slice them, put them in a bowl of cold water to cover.
Drain the potatoes and pat dry. Working in batches if necessary, cook the potatoes in the oil until they are golden brown and crispy, about 3 minutes. Add the garlic if using, along with the potatoes. Remove the potatoes from the oil to a large paper towel-lined bowl and season them with the rosemary and salt, shaking the bowl to distribute the seasonings evenly.
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