Combine corn bread mix, milk, and egg in a large bowl, stirring well.
Pour batter into an 8-inch square baking pan coated with cooking spray. Bake at 400° for 18 to 20 minutes or until a wooden pick inserted in center comes out clean. Let corn bread cool slightly. Crumble corn bread into a large bowl; set aside. Reduce oven temperature to 375°.
Coat a large nonstick skillet with cooking spray; add onion and celery. Cook, stirring constantly, 4 minutes or until tender.
Add vegetables, beaten egg, sage, poultry seasoning, pepper, and cubed bread to crumbled corn bread. Add broth, stirring just until moistened. Pour mixture into a 1-quart baking dish, and bake, uncovered, at 375° for 30 minutes or until golden.