Incise the chest of the Guinea fowl forming a pocket.
Separately cook the Cappelletto by placing it in cold water until it is covered, lighting over low heat and leaving it for 2 hours.
Let it cook and crumble it.
Put it in a bowl with a slice of bread, the egg, the parmesan cheese and mix.
Stuff the Guinea fowl breast pocket with the mixture and close it with a toothpick.
Brown it in a pan with butter and sage and finish cooking in the oven.
Add the mushrooms and cook 20 minutes in the oven at 170 degrees.
Serve with aromatic herbs and pepper.