Saute the onion and celery in butter for about 5 minutes, stirring occasionally.  Blend in the flour, stirring constantly, and heat until mixture bubbles.  

Add milk and broth gradually, continuing to stir while bringing the mixture to a boil.  Cook 2 minutes while stirring until mixture turns into a white sauce.

Take pot off stove, and pour liquid into the peanut butter, mixing until smooth. Return to saucepan, add salt, and heat.  Soup is ready as soon as it is thoroughly heated.