Preheat oven to 375°F (190°C) and lightly grease a 9x13 casserole dish.
Layer broccoli and chicken in the prepared dish, spreading evenly.
In a bowl, mix the sauce: combine cream of mushroom soup, mayonnaise, lemon juice, garlic powder, onion powder, salt, and pepper. Whisk until smooth.
Pour sauce mixture evenly over the chicken and broccoli.
Top with cheese: sprinkle ¾ cup of cheddar over the mixture.
In a small bowl, combine breadcrumbs and melted butter. Sprinkle this topping evenly over the cheese.
Bake uncovered for 25–30 minutes, until bubbly and golden brown on top.
Rest for 5 minutes before serving. Garnish with parsley if desired.