Thinly Slice the onions and add to your jar.
Heat the vinegar, water, sugar, and salt in pot over medium heat. Stir until the sugar and salt has been dissolved. Let cool and pour over onions.
Place in fridge and let sit for 4 hours to overnight before eating. The onions will last up to 2 weeks.
Side note you can keep the liquid and keep adding onions to the jar when you run out.