Slice onions thin with a mandolin slicer or knife.
Pack the onions into a mason jar. Place the jar in the sink.
In a small saucepan, mix water, apple cider vinegar, maple syrup and salt. Bring the mixture to a gentle simmer over medium heat. Stir and remove from heat.
Pour mixture into the jar over onions. Press onions down into liquid with a spoon.
Let cool to room temperature before serving – about 30 minutes. I personally like them cold, so feel free to cover and refrigerate after they are room temp.
Notes
These pickled onions are best when eaten a few days after making them, but will keep for 2 weeks in the refrigerator.