In a large pot, heat butter on medium and sauté shallot and celery until soft. (about 20 min)

Stir in the flour and cook 1-2 minutes longer. Whisk in the broth and milk. Continue stirring until mixture thickens. Add salt and pepper to taste. Remove from heat.

Use in casseroles or thin with milk for a soup. Store in air tight container in refrigerator for up to a week or freeze for later use.