Add thinly sliced red onion to a mason jar and push them down a bit.

In a glass measuring cup combine apple cider vinegar, maple syrup, salt and water.

Microwave for 45 seconds and stir to combine.

Pour mixture over the red onions and let sit at room temperature to cool for an hour, then put the lid on and store in the refrigerator.

Store in the refrigerator for up to 2 weeks!