Optional add-ins - a garlic clove, bay leaf, 1 tsp peppercorns, 1/2 tsp red pepper flakes, 1/4 tsp oregano(These are just suggestions! I would pick one or two, not add them all at once! Or just leave them plain and they're great)
Instructions
Thinly slice the red onion (easiest with a mandoline) and pack into a pint-sized jar.
In a small saucepan, heat the apple cider vinegar, sugar and salt over low heat. Stir occasionally, until sugar and salt have dissolved. Add any optional add-ins to the vinegar mixture.
Carefully pour the vinegar mixture over the onions until the jar is full. Gently press down the onions with a spoon so they are all submerged.
Transfer to the fridge and let sit for at least an hour before using. The onions will taste better after sitting in the fridge overnight. They will last in the fridge for 2 to 3 weeks.