In a medium saucepan, over medium heat, combine apple cider vinegar, honey, red pepper flakes, salt, and black pepper. Stir gently for about 3-5 minutes.

While the pickling liquid heats, slice the red onions thinly using a sharp knife.

Once the pickling liquid is ready, remove it from the heat and place the sliced red onions into a clean glass jar or bowl. Pour the hot vinegar mixture over the onions.

Allow the mixture to cool to room temperature for about 15 minutes, then cover tightly and refrigerate.

For optimum taste, let your pickled onions sit in the refrigerator for at least 1 hour; 2 days are even better!