Preheat the Oven: Preheat your oven to 350°F (175°C) and lightly grease a 9x13-inch baking dish.
Prepare Broccoli and Chicken: If using fresh broccoli, steam or microwave it until tender-crisp (about 3 minutes in the microwave). Chop the cooked chicken into bite-sized pieces if not already shredded.
Mix the Sauce: In a large bowl, combine the cream of chicken soup, sour cream (or Greek yogurt), shredded cheddar cheese, mayonnaise (if using), garlic powder, salt, and pepper. Mix well until smooth.
Layer the Ingredients: Place the cooked chicken and broccoli in the prepared baking dish, spreading them out evenly. Pour the creamy sauce mixture over the top, spreading it evenly to cover the chicken and broccoli.
Add the Topping (Optional): In a small bowl, mix bread crumbs with melted butter until well combined. Sprinkle this mixture over the casserole for a crunchy finish.
Bake: Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the casserole is bubbly and the top is golden brown.
Serve: Allow the casserole to rest for 5 minutes before serving. Enjoy it as is, or pair it with a side salad or crusty bread.