1. Preheat your oven to 400°F. Lightly grease a 9-inch deep-dish pie dish or a similar casserole dish.
2. In a large skillet over medium heat, melt 4 tablespoons of butter. Add the chopped onion and sauté until translucent, about 4 minutes. Stir in the minced garlic and cook for another minute until fragrant.
3. Sprinkle the flour over the onions and garlic, stirring constantly to create a roux. Cook for 2 minutes to remove the raw flour taste, stirring frequently to avoid browning.
4. Slowly whisk in the milk and broth, stirring continuously until the mixture thickens into a creamy sauce, about 5-7 minutes. Be patient here—this step makes the filling luxuriously smooth.
5. Add the Old Bay seasoning, salt, and pepper to taste. Stir in the thawed peas and carrots, then gently fold in the shrimp, fish, and crab meat. Remove from heat.
6. Pour the seafood filling into your prepared dish, spreading it evenly. Sprinkle 1/2 cup of shredded cheddar cheese over the top.
7. In a medium bowl, combine the flour, baking powder, garlic powder, dried parsley, and salt for the biscuit topping. Stir in the buttermilk and 4 tablespoons melted butter until just combined. Do not overmix; the batter should be slightly lumpy.
8. Drop spoonfuls of biscuit batter evenly over the seafood filling, covering as much surface as possible. Sprinkle the remaining 1 1/2 cups cheddar cheese over the biscuits.
9. Bake for 20-25 minutes, or until the biscuits are golden brown and cooked through, and the filling is bubbling around the edges.
10. Remove from oven and brush the biscuit tops with a little more melted butter for a glossy, extra flavorful finish.
11. Let the pot pie rest for 5 minutes before serving to allow the filling to set slightly.

One tip I always share, passed down from Mama Lou herself, is to resist the urge to overmix the biscuit dough. She used to say, “Sofia, let those biscuits have their little lumps. They’ll bake up tender and fluffy that way.” And she was absolutely right.