I have been making this regularly for the past few months. It makes an easy vegetable side dish that goes with everything savory. Noodles, rice, tofu, avocado toast.
Boil salt, sugar and water. Let it cool. Add apple cider vinegar to the brine. Pour it over the sliced onions. Refrigerate for a couple of days and it’s ready to eat. I keep two jars going all the time. The brine is re-useable, but I top off with more brine each time because I use the brine in salad dressings and for avocado toast.
If I had to guess amounts, maybe 2 tablespoons of sea salt, 1 tablespoon of sugar, and 3 cups of water. And a half cup of ACV. I could be totally off here. Just experiment.