Peel and cut the onions into strips (julienne cut).
Add 30 grams of salt to the litre of water (3% brine).
Put the onion that has been cut into julienne strips in the jar.
Cover the jar with brine. It is important that the liquid from the brine completely covers the onions.
Cut a round piece of a big onion and put it on the top, to make some kind of lid. The weight will help the onions stay at the bottom of the jar.
Let the onions ferment for 3-7 days, in a place where they are not in direct sunlight. Make a note of the date when you started fermenting them.
After 3-4 days, you can taste them and if they are to your liking, you can put them in the fridge.