Thinly slice the red onion and pack the slices into a heat-safe glass jar.

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

Pour the hot liquid over the sliced onions in the jar, ensuring all pieces are submerged.

Let the jar cool to room temperature, then secure the lid and chill in the refrigerator before serving.