Preheat oven to its lowest setting. Then line a large shallow baking dish with paper towels and place it in the center of the oven.
Pat the pieces of chicken completely dry with paper towels and season on all sides with 2 tsp of the salt and a few grindings of pepper. One at a time, dip the pieces in 1 C of the flour and turn to coat them evenly. Shake each piece vigorously to remove the excess flour.
Melt 1 1/2 lbs of the lard over high heat in a heavy 12" skillet at least 2" deep and equipped with a tight fitting lid. When melted, the fat should be about 1/2" deep; add more lard if necessary. When the lard is very hot but not smoking, place the pieces of chicken skin side down in the skillet and set the lid on top. Fry over high heat for 5 minutes, turn the pieces of chicken with tongs and continue to fry, still tightly covered, for 4-5 minutes longer, or until the chicken is richly and evenly browned on both sides.
Transfer the chicken to the paper-lined dish and keep it warm in the oven while you prepare the onion gravy. Pour off all but 1 Tbsp of the fat remaining in the skillet and in its place add the onions. Sprinkle them with the remaining 2 Tbsp for flour and, stirring frequently, cook over high heat for 3-4 minutes, or until the onions are soft and golden brown. Stirring them constantly with a spoon, pour in the water in a slow stream and cook until the gravy comes to a boil, thickens and is smooth. Stir in the vinegar, the remaining 1/2 tsp of salt and a few grindings of pepper. Remove the skillet from the heat and taste the gravy for seasoning.
To serve, arrange the chicken attractively on a heated platter and pour the onion gravy over it. Or, more traditionally, mound Southern dry rice in a serving bowl, pour the onion gravy over the rice, and serve the gravy-smothered rice with the chicken.