In a medium saucepan, combine apple cider vinegar, honey, red pepper flakes, salt, and black pepper. Heat on medium and stir for 3-5 minutes until honey is dissolved.

Slice the red onions thinly into ¼ inch rings as the pickling liquid heats up.

Place sliced red onions in a heatproof jar and pour the hot pickling liquid over them, ensuring they are fully submerged.

Let the mixture cool at room temperature for about 15 minutes, then refrigerate. For peak flavor, allow marinating for at least one hour, or up to two days.