Preheat the oven to 350–375°F and grease a 9x13-inch casserole dish.
Combine the main filling ingredients in a large bowl and stir until evenly mixed.
Add the cooked chicken and remaining filling ingredients, mixing thoroughly.
Spread the mixture evenly in the prepared baking dish.
Add the cheese or prepared topping evenly over the casserole.
Bake until hot and bubbling and the topping is golden or cooked through, about 25–35 minutes.
Check the center and bake a few more minutes if needed.
Rest for 5–10 minutes before serving.
Serve warm with simple sides or desired toppings.