In a large bowl or glass measuring cup, combine the salt and water. Stir until the salt is completely dissolved. Set aside.

Pack the onion slices into a clean jar. I usually use the handle of a wooden spoon to press them down.

Pour the brine over the onions, leaving about an inch of headspace. Use a spoon to press the onions, ensuring they are tightly packed. If you find there is more room in the jar, add more onions. Often, the onions will settle after a few minutes, and you can add more.

Place a weight on the onions, making sure they are completely under the salt brine. If any pieces float to the top, either remove them or reposition the weight so they are under the brine.

Put a fermentation lid on the top of the jar (follow the manufacturer's instructions for your fermentation lid).

Let the onions ferment at room temperature out of direct sunlight for about 5-7 days. Around day 5, taste test an onion to see if it is fermented to your liking. If so, swap out the fermentation lid for a regular lid and store in the fridge. If they need more time, continue fermenting and check daily until they have reached your desired flavor.