Start by peeling the medium white onion, then slice it as thinly as possible, about 1/8-inch thick.

In a small saucepan, combine 1 cup of unseasoned rice vinegar, 1 tablespoon of sugar, and 1 tablespoon of kosher salt. Place over medium-high heat and stir until dissolved and boiling, about 3 to 5 minutes.

Transfer your sliced onions into a clean glass jar or a bowl. Pour the hot brine over the onions, ensuring they are submerged.

Allow the jar of onions to sit uncovered at room temperature for about 30 minutes.

After they’ve cooled, seal the jar tightly and place it in the refrigerator.