Heat oven to 450°F. Make and bake cornbread mix as directed on package, using water. Cool 30 minutes; crumble cornbread into large bowl. Reduce oven temperature to 350°F. Spray 11x7-inch (2-quart) glass baking dish with cooking spray. In 10-inch skillet, melt butter over medium heat. Cook celery and onion in butter 10 minutes, stirring occasionally, until tender. Stir in sage, pepper and salt.