Thinly slice the red onion into rounds or half-moons and pack into a clean 500ml glass jar.
In a small saucepan over medium heat, combine apple cider vinegar, water, hot honey, and salt.
Whisk the mixture until the honey and salt are fully dissolved and the liquid is steaming, but not boiling.
Place the red pepper flakes and smashed garlic clove into the jar with the onions.
Pour the hot liquid over the onions until they are completely submerged.
Allow the jar to cool at room temperature for 30 minutes.
Seal the jar and refrigerate for at least 1 hour before use.