In a large stock pot, bring the water, vinegar, salt, and sugar to a low boil. Be sure the sugar has completely dissolved by whisking the mixture together.
Remove from heat. Add the onions and stir to submerge. Set aside.
In the meantime, prepare 4 250 ml jars by washing them thoroughly with hot, soapy water. Be sure to rinse the jars until the soap residue is gone. Set the jars aside. Next, boil a kettle full of water. Place the seals and jar rings into a large bowl. Pour the boiled water over top and let them sit.
Using a tongs, fill the prepared jars with the onion slices. I like to use a metal funnel to avoid any of the onion coming into contact with the rim of the jar. This will help to create a better and safer seal. Fill the jar so that only 1/2 inch of head space remains. Ladle the vinegar and water mixture into the jars to cover hte onions. Remove the funnel and place a hot, sterilized seal on the jar. Screw on the lid until just snug. Be careful! The jar will be hot! Use a kitchen towel to hold the jar in place as you screw on the lid.
Place the jars on a kitchen towel where they will not be disturbed. As the jars cool, you’ll hear a popping sound. This is the hot liquid and air in the jar cooling down and contracting. This will create an air-tight seal and will allow you to store your onions for future consumption.
For best results, I recommend allowing the jars to sit undisturbed for at least 12 hours. With a damp cloth, wipe down the jars, re-tighten the lids, and store in the fridge. Onions will last for up to 3 months.