Preparation:

Grind the celery through a food chopper, being sure to save any juice that runs out of the chopper.
Steep the celery in the milk in a double boiler one-half hour.
Strain out the celery.
Press well to extract all the juice.
Put the liquid back into the double boiler, and heat again to boiling.
Thicken with the flour stirred smooth with a little cold milk.
Stir in oil and salt.