Using a knife, remove the top and bottom of the onion and peel any excess off to reveal the skin of the onion.
Thinly slice the red onion into round strips. Place the slices in a medium-sized wide-mouth jar with a lid.
Stir together the vinegar, sugar, & salt until combined. The mixture will be cloudy and grainy from the coarse salt.
Pour the vinegar mixture over the red onions in the jar. Allow it to sit undisturbed for at least 30 minutes. Enjoy!