With a mandoline, slice the potatoes crosswise into VERY THIN rounds—1/16-inch thick. Place in a large bowl and fill the bowl with cold water. Rinse, moving the potatoes around, then drain. Rinse once more, repeating as needed until the water runs clear, not white (this removes the starches so the chips can crisp). Spread the potatoes onto a layer of paper towels or clean kitchen towels and pat dry. Dry the bowl.
Return the now-dry potatoes to the dry bowl and top with the oil, salt, and a few grinds of black pepper if desired. Toss to coat, ensuring all of the slices are seasoned.
Preheat your air fryer to 325°F. Spread the potatoes in the bottom of the basket in an even layer so that you have no more than two layers of potatoes overlapping. Air fry for 20 minutes, until the potatoes are dark golden at the edges and crisp, tossing them with tongs every 5 to 7 minutes so they cook evenly. If some chips finish earlier than others, remove them to a plate as they are ready. Repeat with remaining potatoes. For two potatoes, I end up doing four batches total.
TO STORE: Store chips in an airtight storage container or ziptop bag for up to 1 week.
TO FREEZE: Freeze chips in an airtight freezer-safe storage container or ziptop bag (remove all the air) for up to 3 months. Let thaw for a few minutes on the counter before serving.