Slice the red onions into thin slices. I like to slice them into full rounds, but you can also slice the onion in half to create a flat surface.
Add the onion slices into the clean jar. Pack in the onions tight.
In a small saucepan, add the water, vinegar, salt, and sugar. Heat on low until the sugar and salt have dissolved.
Pour the warm liquid over the onions in the jar. Use a skewer to poke through the jar to help remove air bubbles.
Place the lid on the jar and allow it to cool before placing it in the fridge.
For best results, leave the jar unopened for 2 days before consuming.
Keep in the fridge, keep well for up to a month.