Soak the grapes in the sweet wine overnight.
The following day, strain the grapes, reserving the wine. Set a large nonstick skillet over moderately high heat. When the skillet is hot add the duck fat or butter and the grapes; sauté, shaking the pan, until they are shiny all over, about 1 minute. Remove to a side dish.
Pour the reserved wine into the hot skillet, bring it to a boil over high heat, and reduce to syrup, 3 to 4 tablespoons. Add the vinegar, stock, peppercorns, and quatre épices. Continue to boil until reduced by half. Add the cream and boil down to a napping consistency, about 3 tablespoons. Add the sauce to the grapes, season with salt and pepper to taste, and adjust the balance of sweet and sour with pinches of sugar or drops of verjus or vinegar.
Wash out the skillet and dry well. (The recipe can be prepared to this point up to 1 hour in advance.)
About 5 minutes before serving, cut the foie gras into 4 slices of approximately equal size, about 2 ounces per slice. Be sure to use a thin-bladed knife, dipped in warm water and wiped clean between each cut. Season each slice with a pinch of salt and pepper. If you like, dip each slice in the flour and shake off any excess.
Set the dry skillet over moderately high heat. When it is hot, add the foie gras slices; quickly lift the pan off the heat for a moment. Return to moderate heat and cook, turning once, for 1 minute on each side. Using a slotted spatula transfer the slices to warmed individual serving plates.
Working quickly, pour off any exuded fat and reserve for some other purpose. Reheat the grapes and sauce in the skillet along with a few tablespoons water or stock to keep the sauce fluid. Bring to a boil and spoon equal amounts of grapes and sauce over each plate of foie gras. Lightly dust each portion with a pinch of fleur de sel and serve at once.
Quatre Épices Despite its name, quatre épices is a mixture of more than four spices. It includes pepper, nutmeg, cinnamon, cloves, ginger, and other spices. You can create your own mixture to taste. I prefer to make up small batches of this seasoning, since ground spices lose their punch in a short time. MAKES ABOUT 1 TABLESPOON 10 whole cloves 1 tablespoon white peppercorns 1 cinnamon stick (preferably the thicker, more pungent cassia cinnamon)
⅔ teaspoon ground ginger ¾ teaspoon freshly grated nutmeg Grind all the ingredients together in a spice mill until powdery. Sieve and store in a tightly capped jar.