Thinly slice your onions and add them to a pint-sized mason jar.
In a small saucepan add your water, salt, and vinegar. Bring to just boiling then turn off the heat.
Pour the hot brine over the onions in the jar, make sure to leave a little room at the top.
Once the pickles have cooled to room temperature, put on a lid and place them in the fridge for a few hours to overnight. Serve & enjoy!