Grease a 9x13-inch casserole pan. Preheat oven to 400 degrees F.
In a large saucepan melt the butter over low heat; add garlic and sauté until fragrant. Whisk in flour and cook 3-4 minutes, whisking constantly. Slowly whisk in milk, mustard and season with salt and pepper, bring to a simmer 4 minutes, whisking occasionally. Add 2 cups of cheddar cheese and parmesan; stir until mostly melted; remove from heat.
Stir in rice and broccoli florets, pour into casserole pan, sprinkle remaining 1/2 cup cheese over top. Bake 30 minutes until bubbly and lightly golden on top. Allow to set 5 minutes before serving.
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Chicken Broccoli and Rce Cassorle (cont.)