Preheat the oven: Heat the oven to 400°F (200°C). Lightly grease a 9 × 13-inch casserole dish.
Cook the aromatics: Melt the butter in a large skillet over medium heat. Add the diced onion and cook for 4–5 minutes, until softened. Add the minced garlic and cook for another 30 seconds.
Make the sauce: Sprinkle the flour over the onion mixture and stir continuously for about 1–2 minutes. Gradually whisk in the chicken broth and milk until smooth.
Thicken the filling: Continue cooking for 3–5 minutes, stirring frequently, until the sauce thickens into a creamy consistency.
Add the chicken and vegetables: Stir in the cooked chicken, frozen mixed vegetables, thyme, salt, black pepper, and parsley. Cook for another 2–3 minutes, just until everything is heated through.
Transfer to the casserole dish: Pour the creamy chicken mixture into the prepared casserole dish and spread it evenly.
Add the biscuits: Arrange the 8 biscuits over the top of the chicken mixture, leaving a little space between them so they can expand and brown properly.
Bake: Bake for approximately 25–30 minutes, or until the filling is bubbling and the biscuits are golden brown and cooked through. Biscuit-topped pot pies are generally done when the filling is bubbling and the biscuit centers are fully baked.
Rest and serve: Remove the casserole from the oven and let it rest for 5 minutes before serving. This allows the creamy filling to thicken slightly.
