Slice the onion and place in a heat-proof container.
Add the rest of the ingredients to a small saucepan and bring to a boil. Lower heat and stir until the sugar and salt have dissolved. Turn off the heat.
Pour the liquid over the onions and press them down with a fork.
Cover and allow to come to room temperature for 30 minutes.
Serve immediately or cover and place in the fridge. These are best served within the first few days, but will keep in the fridge for a couple weeks.
Calories: 122kcal, Carbohydrates: 23g, Protein: 2g, Fat: 1g, Saturated Fat: 1g, Sodium: 3510mg, Potassium: 185mg, Fiber: 2g, Sugar: 17g, Iron: 1mg