To mellow out astringent flavor of onion you first soak sliced onions in boiling water for 20 to 30 seconds. This step will result in par boiling sliced onions first hence softens the onions and tame it’s bite. Follow this step if you can’t handle strong pungent flavors of onion and like to have it mellowed down. Drain blanched onion into colander then add to a jar and pour pickling liquid. This will result in sweeter and less pungent pickled onion.