Preheat the oven to 375 degrees F (190 degrees C). Lightly grease a 9x13-inch baking dish.
Add spaghetti to the prepared baking dish. Drizzle with 2 tablespoons olive oil, then sprinkle with garlic, Italian seasoning, salt, and pepper. Toss gently to coat noodles evenly.
Stir together chicken, chicken broth, marinara sauce, diced tomatoes, onion, bell pepper, and mushrooms in a large bowl. Pour mixture over spaghetti and toss gently to combine. Cover the dish tightly with foil.
Bake in the preheated oven for 25 minutes.
Remove casserole from the oven and stir mixture gently with tongs, pressing noodles down into the liquid. Cover tightly with foil and return to the oven.
Bake for 20 minutes more. Remove foil, sprinkle cheese evenly over the top, and continue to bake, uncovered, until cheese is melted, 15 to 20 minutes more. Let stand 10 minutes before serving. Garnish with parsley.
