For the roulade: Preheat the oven to 425 degrees F. Position a rack in a roasting pan.
Melt 4 tablespoons of the butter in a large skillet over medium heat. Add the celery, fennel and onions and cook, stirring often, until softened, about 5 minutes. Add the sausage and cook until browned through, 7 to 10 minutes. Stir in the parsley and rosemary and cook for about 1 minute. Season with salt and pepper. Deglaze the skillet with a splash of brandy, scraping up any brown bits from the bottom.
Combine the sausage/veggie mix, stuffing mix, chicken stock and egg in a large bowl and stir together. Lay the turkey breast flat and season with salt and pepper. Spread the stuffing mixture in an even layer, then tightly roll the turkey breast. Tie the roll with kitchen twine--approximately 4 to 6 ties evenly spaced
Rub the remaining 4 tablespoons softened butter all over the surface, sprinkle with salt and pepper and put on the rack in the roasting pan. Roast for 30 minutes, then decrease the oven temperature to 350 degrees F and continue roasting until the skin is golden brown and crispy and a meat thermometer inserted into the center of the roulade reaches 155 degrees F, another 1 hour to 1 hour 15 minutes. Remove to a cutting board, cover with foil and let rest while you make the gravy.
For the gravy: Set the roasting pan over a burner over medium heat. Whisk the flour into the drippings to form a paste and cook, stirring constantly, until the roux is golden brown, about 3 minutes. Add the chicken stock, turn off the heat and add the brandy to the pan. Turn the heat back on and cook, whisking constantly, until the gravy thickens, 5 to 7 minutes. Season with salt and pepper.