If you’ve ever had legit Mexican food these will taste familiar. It’s common to soak onions in lime and water to reduce the pungency of them before adding them to tostadas, tacos, and other South of the Border street food. I took the traditional method a step further by adding salt and letting the onions ferment on the countertop for 5 days. The result is nothing short of marvelous, and adds a bright dash of flavor to chicken soup, pozole, tacos, guacamole, sandwiches, and anything else you enjoy onions on while delivering billions of happy probiotic bacteria. 
2 red onions, peeled and thinly sliced into half moons
Juice and zest of 1 lime
1/2 lime, thinly sliced into rounds
4 cups water
2 tbsp. salt
Dissolve salt into water. Layer the bottom of a quart canning jar with lime rounds. Mix lemon juice and zest with onions. Add onions to the quart jar, making sure all juices are included. Top with salt water. Layer an additional layer of lime slices on top of the jar, so they stick out just above the lip of the jar.
Seal the jar. It’s okay (and good) if the water sills out over; this ensures there is less oxygen in the jar.
Let sit in a warm dark place for 3-5 days, breaking the seal every 48 hours to release pressure.
Once the onions are to your liking, refrigerate them to prevent further fermentation.
love fermented onions. going to try my next batch with lime!
You won’t regret it!