Stir together water, vinegar, sugar, salt and any additional spices you like (see notes).
Bring to boil in a small saucepan while you thinly slice the red onion.
Once boiling, add all of the onion, stir to ensure it's fully coated, and turn off the heat.
Allow to cool until warm, and transfer to a jar. Seal tightly, and put in the fridge once room temperature.
Notes
Add spices to enhance the pickle. We like whole coriander seeds, whole peppercorns, chili flakes, and fennel.
This quick pickle will keep well in the fridge, in a sealed container, for many weeks, if it lasts that long. As it cools, the red colour will fully stain the onions a bright pink.
This is a sweet pickle. You can swap the amounts of salt and sugar for a salt pickle for other vegetables, like cauliflower, cucumber, green beans, and more.