Peel the red onion, slice it in half from root to stem, and remove the tough root end. Using your knife or mandoline, cut the onion into incredibly thin, uniform 1/8-inch (3mm) half-moons.

Pack the thinly sliced red onions tightly into your clear glass mason jar.

Place your stainless steel saucepan over medium-high heat. Combine the white vinegar, water, sugar, kosher salt, and black peppercorns. Stir gently until the liquid comes to a rolling boil and the salt and sugar completely dissolve.

Carefully pour the hot pickling liquid directly over the raw red onions in the jar, ensuring they are entirely covered. Use a butter knife to press down on the slices, releasing any trapped air bubbles.

Allow the jar to sit uncovered at room temperature for at least 30 minutes. Watch as the natural daylight illuminates the liquid, transforming it into a light pink translucent brine. Seal and refrigerate once cooled.