Slice and blanch a red onion. Drain and refresh under cold water to stop the cooking. Put the slices in a non-reactive bowl with a a few halved garlic cloves. Grind a half teaspoon or so each of whole cumin seeds, whole black peppercorns, and kosher salt in a mortar and pestle and add to the bowl. Just barely cover the slices with apple cider vinegar and top off with cold water and stir. Taste. You might want to add some salt or more water. Cover and let sit for an hour or so, stirring occasionally. The slices will turn shocking pink. You may need to adjust salt or water or vinegar.
These can be kept for long time in the refrigerator but they are so delicious, you will find things to put them on. They are addictive. Also delicious on fish tacos.