Pour chicken broth in a large saucepan and bring to a low boil. Slowly whisk in flour and then whisk in milk. Continue whisking until the mixture is smooth and thick (about 5 minutes). Set aside.
Saute onions in butter until soft. Add to the broth mixture.
Put broth mixture on low and toss in broccoli, rice, and about 2/3 of the cheese. Stir until the cheese is mostly melted.
Add your seasonings and then transfer into a 9×13″ baking dish. Smooth in the dish and then top with remaining cheese.
Bake at 350ºF (uncovered) for 1 hour.
Notes
Make as directed above, but freeze before baking. Allow the broccoli rice casserole to thaw overnight in the refrigerator and then cook as directed. It reheats nicely as well.