Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked chicken, steamed broccoli, cooked rice, cream of chicken soup, mayonnaise, lemon juice, garlic powder, onion powder, salt, and pepper. Mix until well combined.
Transfer the mixture into a greased 9x13-inch baking dish, spreading it evenly.
In a separate bowl, mix the breadcrumbs with the melted butter until the breadcrumbs are evenly coated.
Sprinkle the breadcrumb mixture over the chicken and broccoli mixture in the baking dish.
Top with shredded cheddar cheese.
Bake in the preheated oven for 30-35 minutes, or until the casserole is bubbly and the top is golden brown.
Let it cool for 5-10 minutes before serving.