To make pickled red onion for your next taco night, all you need is red onion, apple cider vinegar, water, sugar, and salt. After slicing your red onion and placing it in a jar, bring the rest of the ingredients to a boil and stir until the sugar and salt are dissolved; then, pour the liquid over the onion, let it cool before sealing the jar, and refrigerate it for at least two hours.