In a small skillet, saute celery and onion in butter until tender. Transfer to a large bowl. Stir in the cornbread, bread cubes, broth and seasonings. Transfer to a greased 1-qt. baking dish.
For gravy, in a large saucepan, melt butter. Whisk in flour and salt until smooth. Gradually add broth and milk. Bring to a boil. Reduce heat; cook and stir for 2 minutes. Remove from the heat. Stir a small amount of hot mixture into egg; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer.
Spoon half of the gravy over dressing mixture. Layer with turkey and remaining gravy. Sprinkle with bread crumbs if desired. Cover and bake at 350° for 25-30 minutes or until heated through.
Or cover casserole and freeze for up to 3 months.
To use frozen casserole: Remove from the freezer 30 minutes before baking (do not thaw). Bake at 350° for 1-1/2 hours. Uncover; bake 10-15 minutes longer or until bubbly and heated through.