Place the onions, duck fat, garlic, and a pinch each of salt and pepper in an electric slow cooker or heavy earthenware or enameled cast-iron casserole. Cover the slow cooker and cook on low for 12 hours or slow-bake the casserole, covered, in a 250°F oven for 2½ hours. The onions should break down and cook in their own moisture. Add the sugar, and cook, uncovered, for 30 minutes, stirring often. The onions should turn golden and be a little firm to the bite. Let cool, then cover and refrigerate for up to 5 days. (Note that in this recipe, the foie gras is not seasoned in advance, because it would draw out the moisture during the steaming.) The day you plan to serve the dish, let the onions and foie gras return to room temperature. Soak the raisins in the verjus or vinegar for 10 minutes. Reheat the onions and allow them to fry in their own juices until golden brown. Add the raisins with their verjus or vinegar and bring to a boil. Add the white wine and stock and cook over moderately high heat, stirring often, until a thick, rich sauce develops, about 10 minutes. Correct the seasoning with salt and pepper and set aside, covered to keep warm. Preheat the broiler. Bring plenty of water to a full boil in the bottom of a deep steamer (a pasta pot with a steamer basket will work well here). Place the cabbage leaves on the rack and steam, covered, for 2 to 3 minutes, or until they are soft and pliable. Remove and chill under cold running water. Wrap the whole foie gras in the cabbage leaves. Steam for 8 minutes, or until the internal temperature measures 120°F. Remove the packet to a flat surface, unwrap, and let cool slightly. Cut into 4 equal servings and place on a baking sheet; discard the cabbage. Spoon some of the juices from the onion sauce over the foie gras, then run under the preheated broiler about 4 inches from the heat for about 1 minute, just to glaze the top of each slice. Divide the hot onion sauce among 4 warmed plates. Slide the pieces of foie gras on top. Season with a pinch of salt and pepper and serve at once. A larger lobe, weighing about 1 pound, which will serve 6 to 8, will steam in 15 minutes.