Halve the peeled onion through the root and slice into 1–2 mm half-moons. Separate the layers and place in a clean, room-temperature, heatproof 750 ml jar.

Combine vinegar, water, sugar, and salt in a small saucepan. Warm over medium-low heat for about 3 minutes, stirring until dissolved. Do not boil down the liquid. Remove from heat and let heavy steaming subside.

Carefully pour the warm brine over the onion. Press the slices beneath the liquid with a clean spoon, leaving room at the top. Do not pour very hot liquid into cold glass. If needed, use a better-fitting container; do not dilute with extra water.

Let the jar lose its initial heat briefly, then cover and refrigerate promptly, within 2 hours of preparation. Chill for at least 1 hour. Thin slices will be tangy with some crunch; overnight chilling gives more even pink color and a gentler onion bite.

Lift out and drain portions with clean utensils. Keep the remaining onions submerged, covered, and refrigerated at 40°F or colder; use within 1 week. Discard if mold, unexpected bubbling, or an off odor develops. This recipe is not suitable for pantry storage or canning. Do not reuse the brine for a new batch.