Peel the onion and slice it into thin, ⅛-inch slices. Separate the slices into rings and place them in a jar large enough to accommodate all of them (a 1-quart jar should work). Make sure you use a glass jar that can withstand hot, near-boiling water.
In a small saucepan, combine the vinegar, water, and salt. Add the bay leaves and the peppercorns. Bring to a boil over high heat, stirring until the salt dissolves.
Turn the heat off. Remove the saucepan from heat and allow it to rest and slightly cool for about 5 minutes.
Pour the liquid over the onions. With a spoon, press down on the onions to make sure they are well-covered.
Close the lid tightly and allow the jar to sit at room temperature for 2 hours before serving. Alternatively, you can cool it at room temperature for about 1 hour, then refrigerate until ready to use. Refrigerate the leftovers for up to a week.
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Notes
You can use red wine vinegar, white wine vinegar, or cider vinegar. White vinegar changes the color of red onions and turns them pink, as you can see in the photos. So if you prefer a darker color, red wine vinegar is a good option. Apple cider vinegar might cause the red onions to darken a bit too much. But they will still taste good.The leftovers keep well in the fridge, in a sealed glass jar, for about a week. They rarely last that long in our house, though. Since they go so well with so many dishes, we go through them pretty quickly.