Add the Italian sausage to a skillet and sauté for about 5 minutes or until cooked through. As the sausage cooks, use a wooden spoon to break it up into small pieces.
Add the chopped mushrooms, onions, and garlic to the sausage. Turn the heat down to medium and cook for five minutes.
Transfer the mixture to a large bowl too cool.
When the mixture has cooled for about 10 minutes, add the egg yolk, ricotta, mozzarella, Parmesan cheese, salt, and pepper. Stir to combine.