1 large red onion
~1 cup of fresh squeezed lemon juice (approx. 4 large lemons)
Thinly slice the onions.
In an air-tight container, combine with ~1 or 2 tablespoons of salt (to taste) and ~1 cup of fresh squeezed lemon juice. Make sure there is enough liquid to completely submerge the onions.
Cover and let sit in the refrigerator 24 - 48 hrs. Lasts up to 2 weeks in the fridge.