Prep the Oven & Dish: Preheat the oven to 350°F. Grease a 9-x-13-inch baking dish and set aside.
Cook the Pasta: Boil the spaghetti until al dente according to package instructions. Drain and set aside.
Make the Sauce: In a large saucepan, cook the onion and garlic in butter until the onion is soft and translucent.
Stir in cream of chicken soup, cream of mushroom soup, sour cream, 1/2 cup Parmesan cheese, 1/2 cup Mozzarella, chicken broth, salt, and pepper.
Mix well, then add the cooked chicken.
Combine Everything: Stir the creamy mixture into the cooked spaghetti until evenly coated.
Assemble: Pour the mixture into the prepared baking dish. Sprinkle the remaining 1 cup of Mozzarella cheese evenly on top.
Bake: Bake, covered, for 20–30 minutes, or until the cheese is melted and the casserole is bubbling. Remove foil and bake for 10 more minutes if you want a golden top.
Serve & Enjoy: Let cool slightly before serving. Pair with a simple green salad and garlic bread for a complete meal!