Preheat the Oven:
Preheat your oven to 350°F (175°C). Grease a 9x13-inch casserole dish with a little butter or cooking spray.
Prepare the Sauce:
In a medium mixing bowl, combine the cream of chicken soup, mayonnaise, Parmesan cheese, lemon juice, garlic powder, onion powder, and black pepper. Stir until smooth and well combined.
Assemble the Casserole:
Layer the cooked chicken evenly on the bottom of the casserole dish. Top with the cooked broccoli, spreading it out evenly. Pour the creamy sauce mixture over the chicken and broccoli, spreading it out to cover everything.
Add Toppings:
Sprinkle the shredded cheddar cheese evenly over the top of the casserole. In a small bowl, combine the breadcrumbs with melted butter and sprinkle over the cheese for a crunchy, golden topping.
Bake the Casserole:
Bake the casserole in the preheated oven for 25-30 minutes, or until the sauce is bubbling and the top is golden brown.
Serve:
Remove from the oven and let it rest for 5 minutes before serving. Enjoy it as a main dish with a side of rice or a light salad.