In a large saucepan, saute onion in 1/2 cup butter until onion is tender.
Add cheese sauce, milk, broccoli, soup, and rice to onion and butter. Stir until combined.
Transfer mixture to a greased casserole dish.
In a small bowl, combine bread crumbs and 2 Tablespoons of melted butter, stir well to coat. Sprinkle over casserole.
Bake, uncovered, at 350 degrees for 50-60 minutes, until hot and bubbly.