Preheat oven to 375°F. Grease a 15x10x1-inch baking pan; set aside. Remove and discard skin from chicken.
Pour buttermilk into a shallow dish. Finely crush croutons (you should have about 1-1/4 cups). In another shallow dish, combine crushed croutons, lemon-pepper seasoning, and, if desired, celery seeds. Dip chicken in buttermilk, then into crouton mixture to coat. Place chicken in the prepared baking pan.
Bake, uncovered, for 45 to 55 minutes or until chicken is no longer pink (180°F). Do not turn chicken while baking.
To make 1/3 cup sour milk, place 1 teaspoon lemon juice or vinegar in a glass measuring cup. Add enough milk to make 1/3 cup liquid; stir. Let the mixture stand for 5 minutes before using.