Slice a red onion into thin, fine pieces.
Place the onions into a 1-pint mason jar or any other heat-safe container. Put the jar in the sink to prevent any hot vinegar splashes.
Combine the water, vinegars, maple syrup, salt, and pepper flakes in a small saucepan. Heat the mixture over medium heat until it simmers gently, then pour it carefully into the jar, covering the onions.
Press the onions down into the vinegar and eliminate any air bubbles in the jar using a butter knife or spoon. Allow the pickled onions to cool down to room temperature (around 20 to 30 minutes). By then, they will be ready to serve with their delicious pickled flavor.