Start by gathering all your ingredients. Grab a clean mason jar or another airtight container. Thinly slice your red onion into rings. You don’t need to stress about uniformity; just get Thin enough so they absorb the brine quickly.

In a small saucepan, combine the apple cider vinegar, distilled white vinegar, water, smashed garlic clove, black peppercorns, fine sea salt, and your chosen sweetener. If you want a hint of heat, toss in the red pepper flakes. Heat the mixture over medium-low heat, stirring until the salt and sweetener dissolve completely.

Put the sliced onions into your mason jar. You can pack them tightly for maximum flavor absorption. Remember not to overstuff; they still need space to marinate!

Once your brine has reached a gentle simmer and everything has dissolved, pour it hot over your onions in the jar. Ensure they are fully submerged. If they're floating at the top, give them a gentle press down.

Allow your jar to cool to room temperature on the counter. Cover it once it has cooled down and store it in the refrigerator.

For the best flavor, let the onions pickle for at least one hour, although they will be ready to eat in 30 minutes. The longer they sit, the more intense the flavor. Keep them in the fridge; they’ll last for about two weeks.