1. Preheat oven to 350°.
2. Heat a large saucepan over medium-high heat. Lightly coat pan with cooking spray. Add onion to pan; sauté 5 minutes, stirring occasionally. Sprinkle chicken with 1/4 teaspoon pepper; add chicken, meaty side down, to pan. Cook 6 minutes or until browned; turn chicken over. Add broth to pan. Reduce heat, cover, and simmer 15 minutes or until chicken is done. Remove chicken from pan, reserving broth in pan. Let chicken stand 10 minutes. Remove chicken from bones; shred. Discard bones. Place chicken in a large bowl.
3. Bring broth to a boil; cook, uncovered, until reduced to 1/2 cup (about 10 minutes). Add broth mixture to chicken. Melt butter in pan over medium-high heat. Add squash to pan; sauté 3 minutes or until lightly browned, stirring frequently. Add squash mixture, rice, and minced rosemary to the chicken mixture, and toss gently to combine.
4. Place flour in a medium saucepan over medium heat. Gradually add milk, stirring until smooth; bring to a boil. Cook 1 minute or until thick, stirring constantly with a whisk. Remove from heat; let stand 30 seconds. Add 1/4 cup cheese; stir with a whisk until smooth. Add remaining 1/4 teaspoon pepper, milk mixture, and salt to chicken mixture; stir to combine. Spoon chicken mixture into a broiler-safe 8-inch ceramic baking dish coated with cooking spray. Sprinkle remaining 1/4 cup cheese over rice mixture; bake at 350° for 20 minutes or until thoroughly heated. Remove casserole from oven.
5. Preheat broiler.
6. Broil casserole 5 minutes or until golden.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.