Thinly slice the red onion using a sharp knife.

Pack the sliced onions tightly into a clean glass jar.

In a small saucepan, combine water, white vinegar, sugar, and salt. Bring to a gentle simmer over medium heat, stirring until salt and sugar dissolve.

Carefully pour the hot liquid over the onions in the jar, ensuring they are completely submerged.

Let the jar cool to room temperature for about 30 minutes, then seal and transfer to the refrigerator.