Preheat the oven to 350 degrees F (175 degrees C). Spread olive oil on the bottom of a cast iron skillet or large baking dish.
Bring a large pot of lightly salted water to a boil. Cook spaghetti in boiling water, stirring occasionally, until cooked through but firm to the bite, about 12 minutes; drain.
Stir spaghetti, marinara sauce, diced chicken, onion, pepperoni, green pepper, mushroom pieces, and black olives together in a large bowl; transfer into the prepared cast iron skillet. Top with a layer of mozzarella cheese and sprinkle with Parmesan cheese.
Bake in the preheated oven until casserole is bubbling, chicken is cooked through, and cheese topping is lightly browned, about 30 minutes.
| 672 | Calories |
| 27g | Fat |
| 68g | Carbs |
| 37g | Protein |
