01 - Preheat your oven to 375°F (190°C).
02 - In a large skillet or Dutch oven, melt the unsalted butter over medium heat. Add the chopped onion, diced carrots, and diced celery. Sauté for 5 minutes, or until the vegetables begin to soften.
03 - Stir in the all-purpose flour and cook for 1 minute, stirring constantly. Gradually whisk in the chicken broth and whole milk. Continue to simmer and stir until the sauce has thickened, approximately 3–5 minutes.
04 - Add the shredded cooked chicken, frozen peas, salt, black pepper, dried thyme, and garlic powder to the skillet. Stir well to combine all ingredients. Remove the skillet from the heat.
05 - Pour the prepared chicken mixture into a 9x13-inch baking dish.
06 - In a separate mixing bowl, whisk together the all-purpose flour, baking powder, salt, and black pepper for the dumplings. Stir in the melted unsalted butter and whole milk until just combined. Be careful not to overmix. If using, gently fold in the chopped fresh parsley.
07 - Drop spoonfuls of the dumpling batter evenly over the surface of the chicken mixture in the baking dish. Aim for about 12 dumplings.
08 - Place the baking dish in the preheated oven and bake uncovered for 35–40 minutes, or until the dumplings are golden brown and thoroughly cooked through.
09 - Allow the casserole to stand for 5 minutes before serving to allow the flavors to meld and the sauce to set.