Place pour red wine vinegar, water, sea salt, and maple syrup in a non-reactive saucepan over medium heat.
Stirring occasionally, heat mixture until well combined and salt and syrup have dissolved.
Take off heat to cool.
Place sliced onions, and garlic if using, in a mason jar or something with an airtight seal.
Pour cooled mixture into mason jar and put the lid on tightly.
Shake the jar a bit and place in the refrigerator. The onions are ready to use in about 2 hours. The pickled red onion recipe should be good for about 2 weeks. They are ready to use in about 2 hours.