Preheat the oven to 350 degrees F.
Rub each potato with 1/2 teaspoon of the olive oil, and season lightly with salt and black pepper. Place on a heavy baking sheet and bake until very tender, about 1 1/2 hours. Remove from the oven and let sit until cool enough to handle.
Cut the potatoes in half lengthwise. With a melon baller or small spoon, scoop out the pulp into a large mixing bowl, leaving a thin potato border along the skin. Place the potato shells on a baking sheet and reserve the potato flesh for another use.
Salt and pepper the quails inside and out, and place 1 sage leaf inside each. Wrap each quail in prosciutto.
In a large ovenproof saute or braising pan, heat the remaining 1/4 cup of olive oil and melt the 2 tablespoons of butter over medium-high heat. Add the wrapped quail and cook until browned on all sides, 6 to 7 minutes. Remove the quail from the pan. Add 1/2 cup of the white wine, brandy, 4 sage leaves, and 1 tablespoon of rosemary and bring to a boil, stirring to deglaze the pan. Cook until the liquid is slightly reduced, about 2 minutes. Return the quail to the pan and cover tightly. Roast until the quail are cooked through and the leg meat is just pink, 10 minutes, basting once.
Remove from the oven. Place 1 quail into 4 potato halves and cover with the remaining potato halves and place in a gratin dish. Deglaze the saute pan that held the quail with white wine over medium-high heat and, stirring, cook until the pan juices are reduced by half. Remove from the heat and pour over the potatoes. Return to the oven to bake until hot, about 5 to 8 minutes.
In a large skillet heat the bacon fat over medium-high heat. Add the onions and garlic and cook, stirring, for 2 minutes. Add the livers, mushrooms, the parsley, capers, 1/2 teaspoon of salt, and 1/4 teaspoon of ground black pepper, stirring to break up into small pieces. Cook until the mushroom liquor has almost all evaporated and the livers are almost no longer pink. Add the reduced pan juices and the remaining 2 tablespoons of butter and stir to combine. Remove from the heat and adjust the seasoning, to taste.
Remove the quail from the oven. Place 1 quail-stuffed potato on each of 4 plates. Serve with Grilled Radicchio, top with the sauce and garnish with bunches of black grapes and chopped parsley.
Yield: 4 servings