Preheat your oven to 375°F (190°C).
In a large skillet, melt the butter over medium heat. Add the diced onion and celery, and sauté until soft and translucent, about 3-5 minutes.
Sprinkle the all-purpose flour over the sautéed vegetables, stirring continuously for about 1-2 minutes to make the roux.
Gradually whisk in the seafood broth and heavy cream into the roux mixture, stirring until thickened, about 3-5 minutes.
Stir in the garlic powder, Old Bay seasoning, salt, and pepper to taste.
Gently fold in the cooked shrimp, shredded crab meat, and frozen peas and carrots into the creamy filling.
Pour the seafood filling into a 9-inch pie dish, spreading it out evenly.