Slice the potatoes crosswise as thinly as possible with a mandoline.
Rinse the sliced potatoes in several changes of cold water until water runs clear. Drain and transfer to a salad spinner, spin until as dry as possible then blot well with towels—it’s important they are entirely dry before frying.
Heat oil in a heavy-bottomed pot to 375°F.
Fry a handful of potato slices at a time, stirring frequently with a spider strainer, until golden. Remove from oil and drain on a rack set over a baking sheet.
When all potato slices are fried, transfer to a large bowl.
In a small skillet, heat olive oil over medium heat until glossy. Toss in roughly chopped rosemary.
When rosemary starts to sizzle, about 30 seconds, drizzle oil and rosemary over chips in bowl. Sprinkle with sea salt to taste and toss well.