Preheat the oven to 350 degrees Fahrenheit and spray a baking dish with non-stick cooking spray.
In a large skillet melt the ¼ cup unsalted butter over medium-high heat. Add the 1 cup minced yellow onion, 2 ribs chopped celery, and 2 peeled and chopped carrots to the pan and cook, stirring frequently, until the vegetables are tender. About 3-5 minutes.
Whisk the ¼ cup all-purpose flour into the vegetable mixture, then slowly whisk in the 2 cups chicken broth and 1 cup half and half. A gravy should form as the liquid is whisked in.
Stir in the 3 cups cooked and chopped chicken, 1 teaspoon garlic powder, 1 teaspoon dried parsley, ½ teaspoon dried basil, and salt and pepper to taste.
Cook for a few minutes to bring everything together, then pour the chicken and vegetables into the prepared baking dish.
Dumplings
In a bowl, whisk together the 1 ½ cups all-purpose flour, 1 ½ teaspoons baking powder, 1 ½ teaspoons salt, and 1 ½ teaspoons dried parsley. Using a pastry blender or fork, cut the ¼ cup cold, cubed unsalted butter into the flour mixture until it’s evenly distributed.
Pour the ¾ cup + 2 tablespoons whole milk into the mixture to bring the dough together. Drop spoonfuls of the dumpling dough evenly across the top of the chicken and vegetables.
Loosely cover the dish with foil, making sure it doesn't touch the dumplings, and bake for 30 minutes. Uncover the dish and bake for another 10-15 minutes until the dumplings are lightly browned on top.
Remove from the oven and let the casserole cool for a few minutes before serving.
Notes
Storage & Make Ahead Instructions
In the Refrigerator: Store leftovers in an airtight container in the fridge for up to 4 days.
In the Freezer: I don't recommend freezing this recipe.
Make Ahead: Assemble the chicken and vegetables and cover the dish. Keep in the fridge for up to 4 days before making the dumplings, topping the casserole, and baking.