Pack The Jar: Pack the onions tightly into a pint-size mason jar.
Make The Brine: In a small saucepan, combine the water, apple cider vinegar, maple syrup, and salt. Heat the mixture over medium-high heat until it just begins to boil.
Cool The Brine: Remove the saucepan from the heat and let the brine cool for 2 minutes.
Add The Brine: Pour the brine over the onions in the jar, making sure the onions are fully submerged.
Cool The Onions: Let the onions cool to room temperature, then cover the jar and refrigerate it.
Refrigerate And Serve: Refrigerate the onions for at least a few hours before serving. For the best flavor, refrigerate them overnight.