In a medium saucepan, combine the apple cider vinegar, honey, red pepper flakes, salt, and black pepper. Heat over medium heat for about 3-4 minutes until the honey fully dissolves.

Thinly slice the red onions to about a quarter inch thick while the pickling mixture warms. Consider chilling them in the fridge to minimize tears.

Place the sliced red onions in a clean glass jar or bowl. Pour the hot pickling mixture over them, ensuring they are fully submerged.

Let the jar or bowl of onions cool at room temperature for about 15 minutes.

Securely cover the jar or bowl and place it in the refrigerator. Let the onions marinate for at least one hour, preferably 24 hours.

Store in the fridge for up to two weeks. Enjoy as a topping or side condiment.