Preheat the oven to 375°F (190°C). Grease a 9×13-inch casserole dish or baking dish and set aside.

1. In a large skillet, melt the butter over medium heat. Add the chopped onion, celery, and garlic, cooking for about 4-5 minutes until softened.
2. Stir in the flour and cook for another minute, stirring constantly to form a roux. Gradually pour in the chicken broth, whisking to prevent lumps, and bring the mixture to a simmer.
3. Once the sauce has thickened (about 5 minutes), add the heavy cream, thyme, salt, pepper, and garlic powder. Stir to combine, and let the sauce simmer for 2-3 more minutes.
4. Add the cooked chicken, peas, carrots, and corn (if using) to the skillet and stir until well combined. Remove from heat.
5. Pour the chicken mixture into the prepared casserole dish and spread it into an even layer.
6. Open the package of biscuit dough and cut the biscuits into quarters. Arrange the biscuit pieces on top of the casserole, covering the surface completely.
7. Bake in the preheated oven for 30-35 minutes, or until the biscuit topping is golden brown and the casserole is bubbling around the edges.
8. Once done, remove from the oven and let it cool for a few minutes. Garnish with chopped parsley, if desired.
9. Serve and enjoy!