Place the water, vinegar, salt, and sugar into a heavy bottomed sauce pot. Heat over medium heat and whisk periodically, just long enough to dissolve the salt and sugar. We do not need to simmer or boil for this recipe. We just need to dissolve the crystals. Set aside to cool.
While brine is cooling, slice the onion in half, from the root end to the top. Peel away the papery exterior and discard.
Slice each half of the onion, parallel to and working towards the root end, into 1/4 inch thick slices.
Pack the onion slices into clean jars.
Pour brine over onions in the jars.
Seal with lid and allow a day or two for flavors to combine. You'll know they are pretty much ready to go by looking at the color of the onions. They will start out with the typical purple and white appearance. Once the brine does it's job, the onions will be a solid shade of pink.