Presoak a medium-sized or large clay pot, top and bottom, in water for 15 minutes.
Grind the mushrooms and onion fine in a meat grinder, then place in a saucepan, add the butter, and simmer for 5 minutes. Add the flour, blend well, and pour into the presoaked pot. In a bowl, combine the milk, garlic, bay leaves, salt, and pepper, and pour into the pot.
Cover the pot and place it