Heat the oven to 425°F and coat a 9-by-13-inch baking dish with cooking spray.
Add the chicken and potatoes to the dish, then stir in the mustard, broth, garlic powder, salt and black pepper until coated.
Spread the chicken pieces over the potatoes, cover tightly with foil and bake for 30 minutes.
Uncover, stir in the green beans and return the chicken to the top of the mixture.
Bake uncovered for 20 to 25 minutes, until the chicken reaches 165°F, the potatoes yield to a knife and the beans are browned at the tips.
Rest for 5 minutes before spooning the mustardy pan juices over each serving.