Melt 2 tablespoons of the butter in a large skillet over medium-high heat. Once the pan is hot, add the peeled shrimp and scallops. Sauté for approximately 1 minute, allowing the exterior of the seafood to develop a light sear.

Pour the white wine into the skillet to deglaze the pan. Add the heavy cream, chicken stock, Dijon mustard, garlic powder, onion powder, and black pepper. Continue cooking for approximately 1 additional minute.

Remove the shrimp and scallops from the skillet and reserve them while preparing the sauce.

Combine the remaining 2 tablespoons of butter with the all-purpose flour to form a beurre manié. Add this butter-and-flour mixture to the seafood sauce remaining in the skillet.

Reduce the heat to low and allow the sauce to simmer until it reaches a suitably thick and creamy consistency.

Remove the skillet from the heat. Return the seared shrimp and scallops to the sauce, then gently fold in the lump crabmeat until the seafood is evenly coated. Set the mixture aside.

Preheat the oven to 350°F (175°C).

Arrange the two portions of flounder in the bottoms of individual casserole dishes. Add a small amount of water to each dish and lightly season the fish with Old Bay.

Bake the flounder for 10 minutes. Remove the dishes from the oven and carefully drain away any excess water.

Spoon the prepared shrimp, scallop, crab, and cream sauce mixture evenly over the partially baked flounder.

If desired, dust the top with a little additional Old Bay seasoning.

Return the casserole dishes to the 350°F oven and bake for another 25 minutes, or until the casserole is thoroughly heated, bubbling, and lightly golden on top.

Remove the seafood casserole from the oven and allow it to cool briefly before serving. Finish with freshly chopped Italian parsley, if using.