Melt 1/4 cup butter in a heavy bottomed stock pan over medium heat. Add 1/4 cup chopped celery + 1/4 tsp celery salt + 1 tsp sea salt + 1 tsp pepper and saute until celery is beginning to soften, stirring occasionally.
Add 1/4 cup flour to pan and stir to coat. Allow to cook for 1 additional minute.
Add 1/2 cup milk and 1/2 cup broth, whisking to incorporate flour and liquid. Bring to a boil and whisk well until mixture is quite thick–anywhere from 2 to 5 minutes. Remove soup from heat and enjoy as is, or use in your favorite recipe!