Heat the vinegar, water, sugar, and salt over medium heat, and stir until the sugar and salt dissolve.
Slice the red onions as thin as you can. I used a food processor with a slicing disc, but a sharp knife or mandolin works well too.
Break the onions apart and stuff them into the jar. *optional* Add extra seasoning like peppercorns, garlic cloves, and dill.
Let the brine cool slightly, and pour it over the sliced onions.
Allow the jar to cool to room temperature before covering it.
Let it sit for 24 hours, and enjoy.
Pickled onions will last in the fridge for up to 2 weeks.