Salt Freshly ground black pepper 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 mediumhigh, 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.