Slice the red onions thinly (a mandolin works great for consistent slices). Transfer to a mason jar.
In a small saucepan, add the vinegar, sugar, and salt. Heat over medium heat and whisk until the sugar is dissolved. Remove from the heat and stir in the cold water. Pour over the onions.
Use a fork or spoon to push all the onions underneath the brine. Seal with the lid and refrigerate for at least 1 hour. Enjoy!