In a 10- to 12-inch heavy skillet cook sausage over moderately high heat, stirring and breaking up lumps with a fork, until no longer pink. Transfer sausage with a slotted spoon to a large bowl.
Add butter to fat remaining in skillet and cook onions, chopped fennel, fennel seeds, and salt to taste over moderate heat, stirring until fennel is softened, about 10 minutes. Add aperitif, thyme, and tarragon and cook, stirring, until most liquid is evaporated. Add mixture to sausage with corn bread or packaged stuffing and toss to combine well. Season stuffing with salt and pepper and cool completely. Stuffing may be made 2 days ahead and chilled, covered. (To prevent bacterial growth do not stuff turkey cavities ahead.)