Preheat the oven to 350 degrees F. Place a rack in a large roasting pan.
Heat 2 tablespoons of olive oil in a medium (10-inch) saute pan over medium heat. Add the onion and fennel seeds and cook for 6 to 8 minutes, tossing occasionally, until the onion is tender. Add the garlic and cook for one minute only. Off the heat, add the sage and rosemary and set aside to cool.
Meanwhile, spread the turkey breast out on a cutting board, skin-side down. Sprinkle the meat with 4 teaspoons salt and 1 1/2 teaspoons pepper. When the onion mixture is cool, spread it evenly on the meat. Distribute the grated butter on top. Place one layer of prosciutto on top of the butter to cover the meat completely. Starting at the left side of the turkey breast, roll the meat up jelly-roll style to make a compact cylindrical roulade, ending with the seam side down and the skin side up. Tie the roulade with kitchen twine at 1 1/2- to 2-inch intervals as tightly as possible to ensure that it will roast evenly. Slip whole sage leaves underneath each tie of the twine down the center of the roulade.
Place the turkey roulade, seam side down, on the prepared rack in the roasting pan. Pat the skin dry with paper towels, brush the skin all over with 2 tablespoons olive oil, and sprinkle with 1 teaspoon salt and 1/2 teaspoon pepper. Pour the wine and 1 cup of water in the roasting pan (not over the turkey) and roast for between 1 1/2 and 1 3/4 hours, until the skin is golden brown and the internal temperature is 150 degrees F. (This depends on how large the turkey breast is.) Remove from the oven, cover with foil, and allow to rest for 15 minutes. Slice in 1/2-inch-thick slices and serve warm with the pan juices.