Prepare the chicken by shredding it into bite-sized pieces.
In a large mixing bowl, combine shredded chicken with marinara sauce, garlic powder, Italian seasoning, salt, and black pepper.
Grease a 9x13-inch baking dish with olive oil and pour the chicken and sauce mixture into the dish.
Sprinkle shredded mozzarella cheese and grated Parmesan cheese over the top. Add breadcrumbs if using.
Preheat oven to 375°F (190°C) and bake for 25-30 minutes until cheese is bubbly and golden brown.
Let the casserole cool for about 5 minutes before serving.