Peel and thinly slice the red onion.
In a medium saucepan, combine apple cider vinegar, water, sugar, and salt.
Bring the mixture to a gentle boil over medium heat.
Stir until the sugar and salt dissolve completely.
Remove from heat and add the sliced onions.
Add peppercorns, red pepper flakes, and crushed garlic.
Stir well to ensure onions are submerged.
Transfer mixture to a clean jar or container.
Allow to cool to room temperature for about 10 minutes.
Seal the jar and refrigerate for at least 30 minutes before using.