Thinly slice the red onion using a sharp knife or mandoline.
In a saucepan, combine the water, apple cider vinegar, sugar, and salt.
Heat the mixture over medium heat, stirring until the sugar and salt dissolve.
Remove the saucepan from heat and let it cool slightly.
Place the sliced onions in a clean jar or container.
Add peppercorns and a bay leaf to the jar if using.
Pour the vinegar mixture over the onions, ensuring they are fully submerged.
Seal the jar tightly and let it cool to room temperature.
Refrigerate the jar for at least 1 hour before using.
Enjoy your pickled red onions on various dishes!