Preheat oven to 400°F. Lightly coat a 9-by-13-inch baking dish with cooking spray. Heat 1 tablespoon oil in a large high-sided ovenproof skillet over medium-high heat. Add chicken pieces, chopped onion and ½ teaspoon salt; cook, stirring occasionally, until the chicken is beginning to brown, 8 to 10 minutes. Sprinkle with ⅓ cup flour; cook, stirring constantly, for 1 minute. Stir in 2 cups half-and-half and 1 cup broth; bring to a boil, stirring often and scraping the bottom of the pan to remove any browned bits. Boil, stirring constantly, until slightly thickened, about 1 minute. Stir in 2 packages peas and carrots; bring to a simmer, stirring occasionally. Once simmering, remove from heat.

Transfer the mixture to a large bowl; stir in 2 packages rice, ½ cup mustard, ¼ cup honey, ½ teaspoon pepper, ½ teaspoon garlic powder and the remaining ¼ teaspoon salt until evenly combined. Transfer the mixture to the prepared baking dish; spread into an even layer.

Bake until the casserole is bubbling around the edges and the chicken is cooked through (an instant-read thermometer registers 165°F), 15 to 20 minutes. Let stand for 5 minutes before serving. Garnish with scallions, if desired.