Thinly slice the red onion. Break the pieces apart and pack them into a 16 ounce wide mouth jar.

In a small saucepan over medium heat add the water, vinegar, salt and sugar. Stir to combine and bring to a boil, stirring occasionally until the salt and sugar fully dissolve.

Pour about half of the hot vinegar mixture over the onions in the jar. Use a spoon to press the onions down, then pour in the remaining vinegar mixture until the onions are fully submerged.

Let the jar cool for 30 minutes on your counter and then place a lid on the jar and refrigerate.

These are best after 2 to 3 days in the refrigerator, but can be enjoyed right away if you can't wait.