Slice onion as thinly as possible, more like a shave than a cut, across width. (You may need to use a meat slicer or food processor fitted with a 1millimeter blade to slice it this fine.) Heat oil in a small skillet. Working in small batches, fry onions, constantly shaking the pan, just until crispy and golden, about 3 minutes. Remove onions with tongs or a slotted spoon and drain on paper towels.