2 tablespoons butter
1/4 cup finely chopped celery, mushroom or onion
2 tablespoons flour
1/4 teaspoon salt (optional)
1 cup milk
In a heavy saucepan, melt the butter. Saute celery, mushroom or onion. Sprinkle the flour over it and stir to incorporate. Cook over medium-low heat for 2-3 minutes, stirring constantly. Slowly whisk in milk.