01 - Preheat oven to 350°F.
02 - Heat a splash of water in a large nonstick skillet over medium heat. Add onion, carrots, celery, and red bell pepper. Sauté for 5 minutes until softened.
03 - Add chicken breast pieces and garlic. Cook for 4-5 minutes, stirring frequently, until chicken is no longer pink on the outside.
04 - Stir in broccoli and zucchini. Cook for 2 more minutes.
05 - Whisk cornstarch into chicken broth until smooth in a bowl. Pour over chicken and vegetables.
06 - Add thyme, oregano, black pepper, and salt. Stir to combine. Simmer for 2-3 minutes until slightly thickened.
07 - Transfer mixture to a lightly greased 2-quart casserole dish.
08 - Sprinkle breadcrumbs evenly over the top, if using.
09 - Bake uncovered for 25-30 minutes until casserole is bubbly and topping is golden.
10 - Remove from oven, sprinkle with fresh parsley, and let rest for 5 minutes before serving.