1. Salt and pepper the liver slices, then dip them in flour and vigorously shake off the excess.
2. Heat the lard in a heavy 12-inch skillet over high heat until a light haze forms over it, then add the liver slices. Cook them on high heat for 2 or 3 minutes on each side, turning them with tongs. Do not overcook; the liver should be slightly pink on the inside.
3. Remove the liver to a platter, cover it loosely with foil, and keep it warm in a 200° oven while you make the sauce.
4. Pour off all but a thin film of the lard from the skillet and add the onions. Stirring occasionally, cook them for 2 or 3 minutes, or until they are slightly translucent. Pour in the vinegar, raise the heat to high and boil the vinegar almost completely away.
5. Add the chicken stock and bring it to a boil, scraping loose and stirring in any brown bits that cling to the bottom and sides of the pan. Turn the heat to low and let the stock simmer for 2 or 3 minutes.
6. Meanwhile, with a wire whisk, beat the 1 tablespoon of flour into the sour cream in a small mixing bowl, then beat it into the mixture in the skillet. Add the capers and simmer on low heat a minute longer. Pour the sauce over the liver and serve at once.
