Peel and very thinly slice the onion. I like to use my mandoline slicer set on the 1/8 inch setting. If your rings are very large, you can slice them in half.

Heat the vinegar, water, and spices in a saucepan until it comes to a boil. Take off the heat and add the sliced onions, giving everything a stir to combine. Let sit for 5 minutes, then pack the onions into a wide mouthed pint sized mason jar. Pour the liquid, including spices, over the onions. Make sure the onions are completely submerged, if you need more liquid, add a bit more vinegar. Let cool, then cover and refrigerate. Plan to use within a month or so.