Prepare the onions: Peel and thinly slice the red onion. Aim for uniform slices, about 1/8-inch thick. You can use a mandoline for consistent results, or a sharp knife.

Combine pickling liquid: In a small saucepan, combine the apple cider vinegar, water, sugar, and salt. If using, add peppercorns and bay leaf. Heat over medium heat, stirring until the sugar and salt are completely dissolved. Bring the mixture to a gentle simmer, then remove from heat.

Pack the jar: Place the thinly sliced red onions into a clean glass jar. If using fresh herbs like dill or thyme, add them now.

Pour and seal: Carefully pour the hot pickling liquid over the onions in the jar, ensuring they are fully submerged. Leave about 1/2 inch of headspace at the top. Discard the peppercorns and bay leaf if desired.

Cool and refrigerate: Let the jar cool on the counter for about 15-20 minutes. Once it has cooled down, seal the jar tightly with a lid and refrigerate. The onions will be ready to eat in about 30 minutes, but their flavor will deepen over time.