Combine water, apple cider vinegar, red wine vinegar, maple syrup, and salt in a small/medium pot. Bring the mixture to a gentle simmer over medium heat. Allow to simmer until salt has dissolved, stirring occasionally.

Add your sliced red onions to a jar (I find that a 32 oz mason jar is perfect). Carefully pour the vinegar mixture into the jar and gently tap the bottom of the jar onto the counter a few times to remove any air bubbles. You want the brine to cover all of the onions - if it doesn't quite cover all of them, let the jar sit for a few minutes, and then use a fork to carefully press the onions further down into the brine.

Seal the jar with its lid and allow the jar cool to room temperature, then place in the fridge. The flavor will get stronger the longer they sit! You can enjoy these as soon as they are refrigerated to your desired temperature if you'd like, but I recommend letting them refrigerate over night before digging in if you are able!