In a saucepot over medium heat, melt the butter. Stir in the flour and cook until the mixture bubbles.
Whisk in the chicken stock and the milk until smooth. Add the celery and cook, stirring constantly until the mixture thickens to a sauce consistency, about 10 to 15 minutes.
Stir in the Parmesan cheese, salt, and pepper. Taste and adjust seasoning.
Use in your recipe or chill completely before storing in an airtight container in the freezer until ready to use.
Promptly store leftovers in an airtight container in the refrigerator, for up to 3 days.Nutritional values are approximate and based on 1 batch of soup.Adaptations: for cream of mushroom soup, substitute chopped and sautéed mushrooms for the celery.