Melt 2 tablespoons of butter in a large oven-safe skillet or Dutch oven over medium heat. Sauté chopped onion and celery for 5-7 minutes until softened.

Stir in 1/2 cup of all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a roux.

Gradually whisk in chicken broth and milk (and heavy cream, if using), ensuring no lumps. Bring to a gentle simmer, stirring frequently until thickened.

Stir in cooked chicken, frozen mixed vegetables, salt, pepper, and dried thyme. Cook for 3-5 minutes, allowing vegetables to heat and flavors to meld; adjust seasonings as needed.

Prepare Red Lobster Cheddar Bay Biscuits according to package directions, but do not bake them yet.

Pour the chicken and vegetable filling into a 9x13 inch baking dish (if not using an oven-safe skillet). Drop spoonfuls of biscuit dough evenly over the hot filling.

Bake in a preheated oven at 375°F (190°C) for 20-25 minutes, or until the biscuits are golden brown and cooked through, and the filling is bubbly.

While the pot pie bakes, melt the remaining butter (about 6 tablespoons) and stir in the garlic powder from the biscuit mix.

Once baked, remove the pot pie from the oven and immediately brush the warm garlic butter mixture over the hot biscuits. Let it rest for a few minutes before serving.