Heat the oil or oil and margarine in a sauté pan or well-seasoned cast iron skillet over medium-high heat. When hot, add the Brussels sprouts and sprinkle with salt. Sauté, stirring occasionally, until the sprouts are turning golden, about 5 to 10 minutes.
Add ½ cup of the stock or water and bring to a boil. Lower the heat and simmer, covered with a lid left slightly ajar, until the Brussels sprouts are almost completely tender and the stock or water has been cooked away, about 10 minutes.
Remove the lid and add the last ¼ cup of stock or water, the sugar, and the apple cider vinegar. Cook at a lively simmer, stirring occasionally, until the liquid is reduced to a syrup, about 5 minutes. Taste for salt and season with pepper. Serve hot or at room temperature.