Sarsa Criolla (Peruvian Onion Relish)

Slice the onion "a la pluma"-- into paper-thin half-moons, cutting the onion top-to-bottom. This is best if the onion is mild. If using hot pepper, cut it into the finest possible slices-- the shape doesn't matter so long as it's not an eyesore, and you can even mince it if you like.
Put onion (and pepper) into a bowl. Sprinkle over a generous pinch of salt and sort of squish it into the onion with your fingertips. It will change the texture of the onion and tame its spice a little, but the difference will be mild.
Ignore the onion while you squeeze the lime or limes, and pick out the stray seeds.
You may drain off any accumulated salty liquid from the onion, or not. Douse onion with lime juice (and oil, if using), and stir to incorporate everything. If you are using cilantro, add it now.
Serve immediately alongside any savory dishes or all by itself atop rice. If you have leftovers, eat them soon or toss them-- it's gross when it's kept a bit. The magic is all in the first hour or so.
This is the standard relish that accompanies virtually all main-course dishes in Peru. It's great with tamales (as a starter) and on sandwiches (make sure your main filling can stand up to it!) It is crisp, mildly spicy, tart, and salty-- yum! Peru's answer to pickles.
You will find this spelled "zarza," "sarsa," and other ways. It's very rustic and perfectly yummy.