Set your oven to preheat at 375°F (190°C).
In a large skillet over medium heat, melt the butter, then add the diced onion and celery, cooking until they become soft and translucent.
Sprinkle the flour over the softened veggies and stir continuously for 1 to 2 minutes to form a roux.
Slowly whisk in the seafood broth and heavy cream, stirring constantly until the mixture is smooth and thickens.
Season the sauce with garlic powder, Old Bay seasoning, plus salt and pepper to your taste, mixing well.
Gently fold in the cooked shrimp, crab meat, and frozen peas and carrots, then remove the skillet from heat.
Pour the seafood mixture into a 9-inch pie dish or equivalent baking container.
Prepare the Cheddar Bay Biscuits following the package directions, stirring in the shredded cheddar cheese.
Spoon biscuit dough evenly over the seafood filling.
Bake the pot pie in the hot oven for 30 to 40 minutes until the biscuits turn golden and are cooked through.
Allow the pie to rest for about 10 to 15 minutes after baking so the filling can firm up before serving.