Finely slice 2 large red onions lengthwise (about 1 quart) and place in a medium mixing bowl.

Cut 1 jalapeño pepper crosswise into 1/4-inch rings and add to the onions.

Place 1 cup of distilled white vinegar into a small skillet.

Add 2 teaspoons of Kosher salt to the vinegar brine.

Add 1 cup of water to the vinegar brine.

Add 1 cup of sugar to the vinegar brine.

Heat the brine over high heat, whisking until sugar is dissolved.

Bring the brine to a boil.

After the brine reaches a boil, pour it over the onions.

Press the onions down with a spoon until they are submerged.

Place a double layer of paper towels on top of onions and press down until completely saturated in the liquid to keep onions submerged.

Allow the onions to sit until they are softened, and liquid has cooled, about 25 minutes.