Top and tail the onion and remove the papery skin. Cut the onion into halves or quarters, and then cut very thin slices. You could also do this in a food processor if you like.
Place the cut onions in a heat-proof bowl and set aside.
In a small saucepan, add the water, vinegar, salt and sugar, along with the pomegranate molasses, if using.
Whisk well and bring to a boil over medium heat. Boil just until the sugar and salt is dissolved.
Pour the pickling liquid over the onions and stir well. Allow to sit for 15 minutes or so, or until they've cooled enough so you can handle them.
Pour everything (you may not use all the pickling liquid) into your mason jar, top with the lid, and store in the fridge.