Soak the beans overnight.
Simmer in boiling water with the carrot, and onion, then the bay leaf and salt, until they are going to pieces in the water.
Boil rapidly to reduce the liquid but be careful the beans do not burn dry.
Remove the bay leaf, carrot and onion and rub the beans and the liquid through a sieve.
Thin with hot milk and add the butter, cut into small pieces.
Stir well and season, then warm thoroughly.
Serve very hot.