Crumble the bread into the milk and simmer it very gently for 2 hours over a gentle heat, taking care to stir it occasionally so that the bread does not stick to the bottom of the casserole.
During this time cook the brussels sprouts in boiling salted water for 30 to 35 minutes, drain, and mash them.
Mix the mashed brussels sprouts and the bread-and-milk; add pepper.
If you think the soup is too thin, reduce and so thicken it a little by rapid boiling; if too thick, add some milk.
Cut up the butter into small pieces in the tureen, and pour the soup over it.
Serve very hot.