Thinly slice the onions and add them to a glass jar.
In a small pot, add the water, vinegar, salt and sugar and bring to a simmer on medium heat. Stir until the sugar and salt have dissolved. This should take 2-3 minutes. Remove from the heat and let it cool slightly for 5 minutes.
Pour the mixture into the jar covering the onions fully. Let it cool to room temperature before sealing the jar and transferring it into the fridge.
Leave the onions in the fridge for at least 1 hour for them to turn bright pink, then they are ready to serve!