Prepare the dressing a day in advance. Preheat oven to 350 degrees. Spray a 13x9" baking pan with cooking spray. In a large bowl, combine muffin mixes, eggs and milk. Add chopped onions. Pour mixture into pan. Bake about 35-45 minutes or until center is firm. Remove from oven and cool.
Spray a 13x9" oven safe pan with cooking spray. Heat the broth in a saucepan on the stove or in the microwave and bring to a boil. While the broth is heating, crumble cooled cornbread in a large bowl. Add bread pieces. Stir in heated broth and sage; mix until well blended. Pour into prepared pan. Cover with foil and refrigerate. Bake the next day.
Set out the dressing for about 30 minutes before baking. Preheat oven to 350 degrees. Bake for 40 minutes and remove foil to brown the top.