Slice a large red onion through the stem/root. Remove peel on one half and thinly slice. Save the other half of the onion for another recipe.
Add your thin slices of onion to a bowl with the rice wine vinegar.
Sprinkle with granulated sugar, a pinch of kosher salt, and a pinch of MSG if using.
Stir everything together and leave on your counter for at least an hour, stirring occasionally.
At first all the onions will not be inside the liquid, but as things soften and the onion releases more liquid, everything will eventually become submerged.
Add to a container with a lid and store in the refrigerator for a couple of weeks.