Preheat a skillet over medium heat. Dice the onion, garlic, carrot, parsnip, and celery into small, uniform pieces.
Add a lightly spritzed non-stick cooking spray and sauté the onions and garlic until fragrant, about 2 minutes.
Add the diced carrot, parsnip, and celery, cooking for an additional 3-4 minutes until they start to soften.
Dice the chicken breast into bite-sized pieces and add to the skillet. Cook until the chicken is lightly browned and nearly cooked through.
Sprinkle the whole wheat flour over the mixture and stir to coat the ingredients, cooking for about 1 minute to eliminate the raw flour taste.
Pour in the low-sodium chicken broth and low-fat milk, stirring constantly to combine and create a light sauce. Allow the mixture to simmer for 3-5 minutes until it thickens slightly.
Stir in the low-fat Greek yogurt, green peas, and thyme. Season with salt and pepper to taste. Heat through until peas are warmed.
Transfer the mixture into a baking dish and, if desired, top with a light layer of mashed root vegetables or a whole grain crust. Alternatively, serve as a hearty stew-like pot pie filling.
Serve warm and enjoy your lightened-up creamy chicken pot pie with root vegetables!