Place sliced onion in a glass jar or airtight container. Set aside.
Combine hot water, sugar and salt in a small bowl.
Stir until sugar and salt are completely dissolved.
Add vinegar and stir to combine.
Pour the brine over the onion slices. Make sure to completely submerge the onion slices in the brine. Add more water or vinegar if needed.
Seal with a lid and shake to combine.
Let the mixture cool to room temperature on your kitchen counter.
After the mixture has cooled down (about an hour) place the jar it in the fridge.
Allow onions to "pickle" for two days in the fridge before using them to enhance their flavor.
Pickled onions will keep in the fridge for up to 3-4 weeks, if they last that long.