Using a sharp knife, thinly slice the red onion.

In a medium bowl, whisk together the vinegar, water, sugar, and salt until dissolved.

Add the sliced onions to the jar, packing them tightly.

Carefully pour the brine over the onions, ensuring they are fully submerged.

If desired, add peppercorns or a bay leaf for extra flavor.

Seal the jar tightly with the lid.

Place the jar in the refrigerator for at least one hour before serving.

For best results, let them sit overnight to fully absorb the flavors.

Your pickled onions are now ready to use.