Preheat oven to 400°F (200°C).

In a large skillet, melt 1/2 cup of butter over medium heat. Add onion and cook until soft, about 5 minutes. Stir in garlic and cook for 1 more minute. Sprinkle in the flour and cook for 1 minute, stirring constantly. Gradually whisk in chicken broth and milk until smooth. Simmer until the sauce thickens, about 5-7 minutes. Season with salt, pepper, and thyme. Stir in cooked chicken and frozen vegetables. Pour the mixture into a 9x13 inch baking dish.

In a large bowl, whisk together 2 cups flour, baking powder, sugar, and 1/2 tsp salt. Cut in 1/2 cup of cold, cubed butter until the mixture resembles coarse crumbs. Stir in the cheddar cheese. Add the buttermilk and mix with a fork until just combined. Do not overmix.

Drop spoonfuls of the biscuit dough over the chicken filling. Bake for 20-25 minutes, or until the topping is golden brown and the filling is bubbly.

While baking, whisk together 2 tbsp melted butter, garlic powder, and fresh parsley. Brush the garlic butter over the hot biscuits as soon as they come out of the oven. Let the pot pie rest for 5-10 minutes before serving.