Combine red wine vinegar, sugar, mustard seeds, peppercorns, bay leaf, sumac (if using), and salt in a saucepan.

Add apple cider (or water with an extra tablespoon of sugar) to the saucepan.

Bring the mixture to a boil over medium heat.

Cook for a few minutes to reduce the liquid slightly.

While the liquid is reducing, cut the onions in half.

Peel the onion halves.

Slice the onions into thin half-moon shapes.

Remove the saucepan from the heat.

Add the sliced onions to the hot liquid.

Let the onions rest in the liquid for at least 2 hours.

Remove the onions from the liquid before serving.