Pre-heat the oven to 350. Lightly mist a 9x13 baking dish with cooking spray and set aside.
In a large mixing bowl, combine the ground turkey breast with the Parmesan cheese, bread crumbs, parsley, garlic powder, egg, egg white and milk and stir together (or mush with your hands) until mixed. Roll the mixture into about 60 meatballs (roughly 1” diameter).
Heat a tablespoon (or 1 ½ teaspoons if you’re cooking the meatballs in 2 batches) of the oil in a large skillet over medium heat. Place the rolled meatballs into the skillet and cook, stirring occasionally, until the meatballs are browned on all sides and cooked through. Remove the meatballs to your prepared casserole dish.
Place the remaining teaspoon of oil in the same skillet and bring to medium heat. Add the diced onion and the sliced mushrooms and cook, stirring occasionally, for about 5 minutes until the onions are softened and translucent and the mushrooms are softened. Add these vegetables to the meatballs in the casserole dish.
Mix the cream of mushroom soup and the beef gravy together in a mixing bowl and pour over the top of the ingredients in the casserole dish. Use a spoon to mix everything together. Add the pieces of biscuit dough and stir to coat. Bake for 25 minutes. Remove from oven and sprinkle the shredded Mozzarella over the top of the casserole. Bake for an additional 15 minutes until hot and bubbly.