1. Pour the soup into a large sauce pan, then place the pan over medium heat.
2. Scoop out about 1 cup of the broth, and pour it into s cup.
3. Combine the sour cream with the broth, and mix until well combined.
4. Add the frozen peas & carrots, and the mushroom into the pan, and stir.
5. Next, pour the sour cream mixture into the pan.
6. Combine the ingredients, then sprinkle 1/4 cup of the mozzarella, and 1/4 cup of the cheddar cheese.
7. Stir the ingredients, then top the tetrazzini with the remaining cheese and the parsley flakes.
8. Serve and enjoy!