RINSE THE ONIONS IN A strainer, just to separate them and remove extra ice. Blot them on paper towels. In a large, heavy skillet, heat the oil over high heat. When it is hot, add the onions and sugar and continue cooking for 5 minutes, shaking often, until the onions are lightly colored. Add the beef stock, adjust the heat to medium- high, and continue cooking until the liquid is almost completely evaporated, about 20 to 25 minutes. Stir in the vinegar, cook for 30 seconds, season with salt and pepper, and serve.