Begin by peeling and thinly slicing the red onions. Use a sharp knife or a mandoline slicer to achieve consistent slices for the best results. Once cut, separate the individual onion rings and place them in a clean glass jar or airtight container.
In a small saucepan, combine the white vinegar, water, sugar, salt, and any optional spices you desire. Bring the mixture to a gentle boil over medium heat, stirring occasionally until the sugar and salt fully dissolve.
Once the pickling liquid is ready, carefully pour it over the sliced onions, submerging them fully. Allow the onions to cool to room temperature before sealing the jar or container and placing it in the refrigerator. Let the flavours meld for at least 24 hours, although longer marinating times will intensify the taste.
Once the pickled onions have had time to marinate, they can be enjoyed in various dishes. Their tangy flavour and crisp texture make them a versatile addition to salads, sandwiches, tacos, or even as a standalone condiment.