Pre-heat the oven to 350. Lightly mist a 9x13 baking dish with cooking spray and set aside.

Melt the light butter in a medium saucepan over medium-low heat. Add the flour to the butter one tablespoon at a time and stir to combine. Add the milk a little at a time and whisk the flour mixture into the milk until smooth. Add the salt and pepper and stir to combine. Turn the heat up to medium and add the cheddar cheese to the milk mixture. Whisk together for 5-6 minutes until smooth and thick.

Combine the rice, chicken and broccoli florets in the prepared baking dish. Pour the cheese sauce over the top and stir the mixture together until well combined. Spread evenly across the casserole dish.

Place the bread crumbs in a dry skillet over medium heat and brown bread crumbs for a few minutes, stirring frequently until golden. Sprinkle the toasted bread crumbs over top of the casserole and bake in the oven for 30-35 minutes until hot and bubbly.