Sterilize mason jar as well as weight and pipe lid, let completely cool.
Mix salt into water, then stir until salt is dissolved.
Thinly Slice onions. Pack onions firmly into jar, leaving 3/4 inch head space.
Pour room temperature saltwater brine over onions, ensuring they are fully submerged.
Place glass weight on top of the onions. If there is too much water for the weight to be even with the rim of the jar, simply pour out just enough water for weight to fit. Do not expose any of the sliced onions- they need to be fully submerged.
Top with the pipe lid and screw on a jar ring to keep it in place.
Set out of direct sunlight, in room temperature for 4-7 days until the fermentation is complete.
When fermention is complete, remove weight and pipe lid and cover with airtight lid. Store in fridge.