Prepare Cornbread: Preheat oven to 450 degrees F. In 12-inch cast iron or other heavy oven-proof skillet, melt margarine on medium-high. Add celery and onion and cook 3 to 4 minutes or until just tender. Remove from heat; stir in sage.
Meanwhile, in large bowl, with wire whisk or fork, stir together flour, cornmeal, sugar, baking powder, baking soda, and salt. In medium bowl, whisk buttermilk and eggs until well blended.
Pour buttermilk mixture into flour mixture. Stir just until blended. Add celery mixture and stir until evenly combined; do not wipe out skillet.
Grease same skillet. Pour batter into skillet and spread evenly. Bake 20 minutes or until toothpick inserted in center comes out clean. Cool 15 minutes. Invert onto wire rack; remove from pan and let cool completely. Cornbread can be wrapped tightly in plastic wrap and kept for up to 3 days at room temperature.
Prepare Stuffing: Preheat oven to 350 degrees F. Grease shallow 3-quart ceramic or glass baking dish.
In large bowl, whisk milk and eggs until well blended. Finely chop cooled cornbread. Transfer to bowl with milk mixture, along with bread crumbs, 1/2 teaspoon salt, and 1/2 teaspoon freshly ground black pepper. Stir until well combined.
Transfer to prepared baking dish and cover with foil. Bake Stuffing 35 minutes. Uncover and bake 35 minutes longer or until toothpick inserted comes out with only a few crumbs attached.
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