Remove green tops from fennel; snip enough of the tops to make 1 tablespoon. Set aside. Cut fennel bulb into wedges, removing the core. Coarsely chop fennel (about 1-1/4 cups); set aside.
Press thawed spinach in a colander to remove liquid. Transfer spinach to a large mixing bowl; set aside. Cook turkey sausage in a skillet until brown. Drain off fat. Add sausage to spinach.
Cook fennel and onion in hot margarine or butter in the same skillet until tender. Stir in snipped fennel tops, sage, pepper, and salt. Add fennel mixture to sausage mixture. Stir in bread cubes. Drizzle with enough broth to moisten; toss gently to coat.
Use to stuff a 12- to 14-pound turkey. Spoon remaining stuffing into a casserole. Cover and chill casserole until ready to add to the oven. The last 30 to 45 minutes of turkey roasting, add the casserole and bake until heated through.