Slice the red onions thinly and place them in a large jar or bowl.
In a saucepan, combine apple cider vinegar, water, sugar, and salt.
Heat the mixture over medium heat until the sugar and salt dissolve.
Add black peppercorns, mustard seeds, and the smashed garlic clove.
Bring the mixture to a gentle simmer, then remove from heat.
Pour the hot pickling liquid over the sliced onions.
Ensure the onions are fully submerged in the liquid.
Let the mixture cool to room temperature.
Once cooled, cover the jar and refrigerate for at least 1 hour.
For best flavor, allow the onions to pickle overnight!