Arrange cornbread in single layer on sheet pan and place in oven. Bake until lightly toasted. Remove from oven and reserve.
Melt butter in large sauté pan over medium heat. Add onion, apple, and celery and cook until tender. Add sausage and cook for an additional 3 minutes. Remove from heat.
In large bowl, whisk together eggs, chicken stock, herbs, salt, and pepper.
In another large bowl, combine sausage mixture with cornbread. Pour egg mixture on top and gently combine.
Pour into 9×13-inch baking dish. Cover dish with foil and place in oven. Bake covered for 30 minutes, then remove foil and continue to bake for an additional 20 minutes or until brown.