Gather all ingredients before starting the recipe.
Pack sliced onions into a medium sized mason jar with a tight-fitting lid.
Combine 1/2 cup water, 1/4 cup distilled vinegar, 1/4 cup apple cider vinegar, 1 1/2 tbsp maple syrup, 1 1/2 tsp kosher salt and 1/4 tsp Asian chili sauce (hot) in a heat proof measuring cup and stir to combine. Heat in microwave on high until the mixture comes to a boil.
Pour mixture over the onions, pressing the onions until all are submerged. Allow to come to room temperature and serve. Refrigerate the remaining onions to serve at another time.