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Peel and thinly slice the red onions into half-moon shapes. In a non-reactive bowl or measuring cup, combine the red wine vinegar, salt, and sugar. Stir until the salt and sugar are fully dissolved.
Place the sliced red onions in an airtight container. A glass jar or plastic container with a tight-fitting lid works well.
Pour the prepared pickling liquid over the red onions in the container, ensuring that the onions are fully submerged.
Seal the container with its lid or cover it tightly with plastic wrap. Place the container in the refrigerator.
Allow the onions to marinate in the refrigerator for about 1-2 hours or longer for a stronger flavor. The longer they marinate, the more pronounced the pickled flavor will be.