Preheat oven to 425 °F (220 °C). Grease a 9×13-inch baking dish.
In a large bowl, combine orzo, chicken broth, onion soup mix, shredded chicken, spinach, sour cream, and cream of mushroom soup. Stir until well combined.
Spread mixture evenly in the prepared dish. Cover tightly with foil.
Bake for 30 minutes. Remove foil and sprinkle mozzarella cheese evenly on top.
Bake uncovered for 15 minutes, until cheese is melted and bubbly.
Let cool slightly before serving.
Video
Notes
Substitutions: Swap cream of chicken soup for mushroom. Use kale instead of spinach. Try Monterey Jack or provolone instead of mozzarella.
Storage: Refrigerate up to 4 days; freeze baked casserole up to 2 months.
Reheat: Warm covered in a 350 °F oven until hot. Add a splash of broth if needed.