In a small bowl. combine the hot water, cider vinegar, sugar, and salt. Stir until the sugar and salt have dissolved. Pour the liquid over the onions - enough to cover them fully. Place the lid on and refrigerate overnight, at least. Keep refrigerated. The red color on the onions will fade and the onions will turn a light pink. Serve on burgers, hotdogs, bbq sandwiches, by themselves, you name it! They're even great on tacos!