Preheat your oven to 204 °C (200 °C). Melt 30 ml of unsalted butter in a large oven-safe skillet or Dutch oven over medium heat.

Add the diced onion, carrots, and celery to the skillet. Cook, stirring occasionally, until the vegetables begin to soften, about 5 to 6 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.

Sprinkle the flour over the vegetables and stir continuously for 1 to 2 minutes to cook out the raw flour taste. Slowly pour in the chicken broth and heavy cream, whisking constantly to prevent lumps from forming.

Bring the sauce to a gentle simmer. Stir in the cooked chicken, thawed peas and corn, dried thyme, salt, and black pepper. Let the mixture simmer for 3 to 5 minutes until it thickens into a rich gravy, then turn off the heat.

In a medium bowl, combine the biscuit baking mix, shredded cheddar cheese, and garlic powder. Add the milk and stir with a wooden spoon or fork just until a soft dough forms. Do not overmix.

Drop spoonfuls of the cheesy biscuit dough evenly over the warm chicken filling in the skillet, leaving small gaps for steam to escape during baking.

Place the skillet in the preheated oven and bake for 15 to 18 minutes, or until the biscuit tops are golden brown and the filling is bubbling around the edges.

While the pot pie bakes, melt the remaining 30 ml of butter and stir in the chopped fresh parsley. Immediately after removing the skillet from the oven, brush the melted herb butter generously over the hot biscuits. Let cool for 5 minutes before serving.