To make the quick pickled onions, press the sliced onions into a 1-pint (473 ml mason jar) or similar heat-safe container. Place the jar in the sink, as this will help to catch any splashes of hot vinegar later.
In a small saucepan, combine the water, white vinegar, apple cider vinegar, pure maple syrup, and sea salt. Bring the mixture to a gentle simmer over medium heat, then carefully pour the heated mixture into the jar over the onions.