Melt butter for the roux in a thick saucepan or in the top of a double boiler. Cook finely chopped onion and celery in it over a low heat until onion is soft but not browned. Remove pan from heat, stir in flour, return to heat, and cook gently for 3 to 5 minutes, stirring constantly, until flour is cooked through. Add a quart of milk, heated to boiling point, and cook over water stirring vigorously. As the sauce begins to thicken, add the remainder of the milk, stirring constantly with a wooden spoon until sauce bubbles. Add thyme, bay leaf, pepper, and nutmeg to taste. Simmer gently for 15 minutes. Strain through a fine sieve and dot with butter.{$7e}(Original recipe for 4)
