Thinly slice the red onion. I like to cut the onion in half first and then slice into half moons, but you can leave it whole if you prefer rings.
Add sliced onions to a small pot. Pour the red wine vinegar, water, orange juice, and salt over the onions.
Let everything simmer over medium-low heat for 5 minutes.
Turn off the heat and allow the mixture to cool slightly. Then, pour the onions and the liquid into a mason jar or other glass container. Allow them to come to room temperature and then place in the fridge. They’re even better the next day and will keep in the fridge for about a week.
Notes
I'd recommend using a glass container since the gorgeous pink pickling liquid will stain plastic. I use these wide mouth mason jars to store the onions. The wide mouth makes for easy pouring and the jar is pretty enough to pop on the table when it's time to eat.