Preheat oven to 400°F. Heat 1 tablespoon oil in a large oven-safe skillet over medium-high heat. Add chicken pieces, chopped onion, ¾ teaspoon salt and ½ teaspoon pepper; cook, stirring often, until the chicken is beginning to brown and the onion is softened, about 5 minutes. Sprinkle with 2½ tablespoons flour and ¼ teaspoon garlic powder; cook, stirring constantly, for 1 minute. Stir in 1 cup broth; bring to a boil over medium-high heat, stirring often and scraping the bottom of the pan to remove any browned bits. Stir in 1 package rice, 2 tablespoons lemon juice and ¼ cup dill until combined.

Bake, covered, until the casserole is bubbling around the edges and the chicken is cooked through (165°F), 15 to 20 minutes. Stir in 1 cup peas, ¼ cup half-and-half and the remaining ¼ cup dill until well combined. Sprinkle with 2 tablespoons lemon zest. Garnish with additional dill, if desired.