Slice the onions thinly with a sharp chef's knife or a mandoline.
In a large glass bowl or jar, add all ingredients and toss to combine making sure the onions are coated through.
Refrigerate for at least 30 minutes before serving. Shake or mix the onions at least once while they cure. This will ensure all of the onions are covered in the juice and will pickle evenly.
Taste before serving and add salt if needed.
