Peel the red onion and slice it thinly. You can use a knife or a mandoline slicer for even, thin slices.
In a small saucepan, combine the white vinegar, water, sugar and salt. Bring the mixture to a simmer over medium heat, stirring until the sugar and salt have dissolved. *Be careful, because these fumes will irritate your eyes if you get your face too close. Remove the pan from heat and let the mixture cool for a few minutes.
Place the sliced red onion in a clean jar or airtight container.
Pour the warm pickling liquid over the sliced onions, making sure they are fully submerged. If needed, use a spoon to press the onions down. The pickling liquid should cover the onions completely.
Let the pickled onions cool at room temperature for about 15-30 minutes. Then, cover the jar or container and place it in the refrigerator.
You can use them now an they're great, but they get better if you allow the pickled onions to marinate in the refrigerator for at least 1 hour. The longer they sit, the more the flavors will develop.
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Notes
The pickled onions can be stored in the refrigerator for up to two weeks.They are amazing on EVERYTHING from tacos to pulled pork, nachos, bbq chicken, salmon, burgers, avocado toast, sandwiches, eggs, mexican casseroles, nourish bowls, etc.