Cut the onion in half root to stem, then remove the skin.
Put the onions cut-side down on a cutting board, then make equally-spaced horizontal cuts from top to bottom.
Next, make vertical cuts through the onion halves top to bottom, trying to be as equally spaced as you were with the horizontal cuts to make it so that when you cut the onion, it will make tiny squares.
Holding the root end, cut the onion vertically to dice it into tiny cubes. Put the diced onion into a non-reactive container like a glass mason jar, add a pinch of salt and a few grinds of fresh black pepper, mix, add the vinegar to cover, top with a plastic or other non-reactive lid, and allow to rest at room temperature for a day or two.
The onions can be used within a few hours of soaking in the vinegar, but will improve in flavor over the next few days. Finished pickled onions can be stored either in the fridge or on the counter and are shelf-stable.