2 cups cooked chicken, cubed
2 cups cooked rice
1 1/4 cups Italian cheese blend, shredded
1 1/4 cups sharp cheddar, shredded
1/2 cup onion, diced
1 head broccoli florets, chopped (about 3 cups)
1/2 cup celery, diced
1 tablespoon vegetable oil
3 tablespoons butter, divided
3 tablespoons flour
2 cups milk, room temperature
1/2 cup Greek yogurt
1 teaspoon salt
1/2 teaspoon ground pepper
1 cup cornflakes
Preheat oven to 350°F. Coat 9-by-13-inch baking dish with cooking spray.
Warm vegetable oil in skillet over medium-high heat. Sauté onion, broccoli and celery until vegetables are soft and broccoli is bright green. Transfer to baking dish and spread around evenly.
Add rice and chicken to vegetables in baking dish.
In the same skillet, melt 2 tablespoons of butter in medium saucepan over low heat. Whisk in flour, and continue stirring for two minutes.
Add milk slowly, continuously whisking until sauce thickens, then remove from heat. Add cheeses and Greek yogurt. Stir until melted. Pour cream sauce over top of rice, chicken and vegetables.
Melt remaining 1 tablespoon of butter. Toss over cornflakes until moistened and sprinkle over casserole.
Cover with foil and bake 25 minutes. Uncover, and bake an additional 5 to 7 minutes for a golden, crispy topping.