Thinly slice the red onion and pack it tightly into a clean glass jar along with the chopped fresh cilantro.

In a small saucepan, combine the white vinegar, water, granulated sugar, and kosher salt over medium heat until the sugar and salt completely dissolve.

Carefully pour the hot vinegar brine over the onions and cilantro in the jar, ensuring they are fully submerged.

Let the mixture cool to room temperature for about thirty minutes before securing the lid and transferring to the refrigerator.