Thinly slice onion using a sharp knife or mandoline slicer. Place thinly-sliced onions in a mason jar.
Boil 1 cup of water or use an electric kettle to heat water.
Pour the hot water, vinegar, sweetener and sea salt into the jar over the onions. Screw the lid on tightly and shake the onions until everything is mixed. If necessary, press onions down so they are submerged under the vinegar mixture.
Let the onions marinate for about 30 minutes.
Enjoy immediately or refrigerate the pickled onions for up to 2 weeks. For the best flavor, let them brine for 24 hours.
Recipe by Raw Manda at https://rawmanda.com/quick-pickled-onions/