Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked rotini pasta, shredded chicken, broccoli, bell pepper, mushrooms, and ½ cup of the cheddar cheese.
In a separate bowl, whisk together the cream of chicken soup, milk, garlic powder, onion powder, Italian seasoning, salt, and pepper until smooth.
Pour the soup mixture over the pasta and chicken mixture, stirring until everything is well coated.
Transfer the mixture to a greased 9x13-inch baking dish and spread it evenly.
Sprinkle the remaining ½ cup of cheddar cheese on top. If using breadcrumbs, mix them with olive oil and sprinkle over the cheese for added crunch.
Bake in the preheated oven for 25-30 minutes, or until the casserole is bubbly and the top is golden brown.
Let it cool for 5-10 minutes before serving.