Preheat the oven to 350°F. Brush the bottom and sides of a 12 inch cast iron skillet or similar with olive oil. Set aside.
Cook the tortellini in salted water per the package instructions. Drain well.
Meanwhile, drizzle a large skillet with olive oil. Over medium high heat cook together the Italian sausage, onion, pepper and mushrooms until no pink remains in the sausage. Drain excess fat from the pan.
Place tortellini in a large mixing bowl. Add the cooked sausage, marinara sauce, cubed mozzarella, 1/3 cup Parmesan cheese, pepperoni, olives, 2 Tbsp parsley, Italian seasoning, garlic salt, and red pepper flakes. Mix well then pour into the skillet. (Set aside 15 pepperoni slices and additional mushroom slices for the top.)
Arrange remaining pepperoni and mushrooms on top. Sprinkle with shredded mozzarella and 2 Tbsp grated Parmesan cheese.
Bake for 35-40 minute or until bubbly and the cheese is golden.
Garnish with remaining parsley and serve.