Pre-heat oven to 375°F.
In a non-stick skillet, over medium heat, melt butter.
Add onion and celery; cook for 5 minutes.
Season with a little salt and pepper, or to taste.
Transfer mixture to a 9-inch x 13-inch baking pan; spread evenly.
Add chicken to baking dish and spread evenly.
In a medium bowl, whisk together flour and milk; slowly pour over chicken, ensuring all chicken is covered; do not stir.
Using the same medium bowl, stir together chicken broth, chicken soup and sage.
Slowly pour into baking pan; do not stir.
Place in oven and bake, uncovered, for 35 to 45 minutes.
Remove from oven and let sit 5 minutes before serving.