Peel and slice red onions into thin rings, about 1/8-inch thick. Pack into clean glass jars with bay leaves and peppercorns.

Combine vinegar, water, sugar, and salt in a saucepan over medium-high heat. Stir until sugar and salt dissolve completely.

Bring mixture to a gentle boil, then remove from heat immediately.

Pour hot brine over onions, ensuring they're completely submerged. Press down with a spoon if needed.

Cool to room temperature for 30 minutes, then seal and refrigerate for at least 1 hour before serving.