Peel and finely chop the onion. Trim the stems of the shiitake mushrooms and thinly slice them. Thinly slice the white mushrooms.
Trim the crusts from the bread and put the bread and milk in a small bowl. Mash the bread with your fingers until you have a smooth pulp. Put it in a mixing bowl with the ground beef, egg, nutmeg, and Parmigiano. Season with salt and