Preheat oven to 375°F (190°C).
- Prepare stuffing mix according to package directions using chicken broth and butter instead of water. Set aside.
- Steam or microwave broccoli until just tender. Drain excess water.
- In a large bowl, mix cooked chicken, broccoli, cream of chicken soup, sour cream or mayo (if using), salt, pepper, and optional seasonings.
- Spread the mixture evenly in a greased 9×13-inch baking dish.
- Top with shredded cheddar cheese if using.
- Spoon the prepared stuffing evenly over the top of the casserole.
- Bake uncovered for 30–35 minutes until hot and bubbly with a golden stuffing top.
- Let rest for 5 minutes before serving.