Preheat your oven to 400°F.
In a large skillet over medium heat, melt the butter.
Add the diced onion and celery, cooking until softened, about 5 minutes.
Stir in the flour and cook for 1 minute.
Gradually whisk in the seafood stock and heavy cream, bringing the mixture to a simmer.
Add the garlic powder, dried thyme, salt, and pepper.
Stir in the cooked seafood, peas and carrots, and diced potatoes. Cook for an additional 5 minutes until the mixture thickens slightly.
Transfer the seafood mixture into a 9-inch pie dish.
In a separate bowl, prepare the Cheddar Bay Biscuit mix according to package instructions, adding the shredded cheddar cheese and milk.
Drop spoonfuls of the biscuit mixture over the seafood filling, covering it as evenly as possible.
Bake in the preheated oven for 20-25 minutes, or until the biscuit topping is golden brown and cooked through.
Let cool for 5 minutes before serving.