Preheat oven to 400°F.
Heat oil in a large high-sided ovenproof skillet over medium-high heat. Add chicken and chopped onion; cook, stirring occasionally, until the chicken is no longer pink on the outside, about 7 minutes. Sprinkle the mixture with flour and cook, stirring occasionally, for 1 minute. Add milk to the pan and bring to a boil, stirring frequently. (Be careful, the pan will be very full.) Boil, stirring, for 1 minute.
Meanwhile, place broccoli and water in a microwave-safe container. Cover and microwave on High until the broccoli is tender, about 3 minutes. Drain.
Remove the pan from the heat and carefully stir in rice, cheese, dry mustard, garlic powder, salt, pepper and the broccoli. Sprinkle with crispy onions.
Bake the casserole until bubbling at the edges, about 10 minutes. Let stand for 5 minutes before serving.
