In a medium saucepan, combine apple cider vinegar, honey, red pepper flakes, salt, and black pepper. Heat over medium, stirring until honey dissolves.

While the liquid warms, slice red onions thinly and place in a clean jar.

Once the pickling liquid is hot, pour over sliced onions, ensuring they are submerged.

Let cool to room temperature before sealing and refrigerating.

Allow to marinate for at least 1 hour, ideally overnight, before serving.