Place the sliced red onion in a very clean mason jar along with the black peppercorns, cumin seeds, Mexican oregano, and garlic.
In a small pot, heat the vinegar and water. Bring to a boil, then add the sugar and salt, stirring to dissolve, then remove from heat.
Pour the hot mixture over the red onions and spices, adding more vinegar if needed to completely cover the onions. Cover, shake, open the lid, and set on the counter for an hour or two or until the mixture is at room temperature. Once cool, cover and refrigerate.
The onions will begin to soften and turn a beautiful hue after several hours. As they age they will continue to improve in taste although the color will fade over time.
Find it online: https://frugalhausfrau.com/2017/06/04/pickled-red-onions/