Step 1: Gently Cook the Seafood
Pat all seafood (scallops, shrimp, fish) dry with paper towels. Season lightly with salt and white pepper. In a large skillet over medium-high heat, add olive oil or butter. Quickly sear the scallops for 45-60 seconds per side just to get a touch of color; remove to a plate. Add the shrimp to the skillet and cook for 1-2 minutes per side until just pink; remove to the plate. Finally, add the white wine to the skillet and bring to a simmer. Add the chunks of white fish and poach gently for 2-3 minutes until just opaque. Remove fish with a slotted spoon. Reserve all the flavorful cooking liquid in the skillet. Gently fold in the crab or lobster meat (if using) with the other cooked seafood.

Step 2: Build the Flavor Base for the Sauce
In a large saucepan or Dutch oven over medium heat, melt the 6 tablespoons of butter. Add the chopped leek (or onion) and celery. Cook, stirring often, for 6-8 minutes until softened but not browned. Add the minced garlic and cook for 1 more minute until fragrant. Sprinkle the flour over the vegetables and stir to coat, creating a roux. Cook, stirring constantly, for 2 minutes to cook out the raw flour taste.

Step 3: Create the Velvety Cream Sauce
Gradually whisk in the seafood stock, followed by the heavy cream and milk, ensuring no lumps form. Whisk in the reserved cooking liquid from the seafood skillet. Bring the mixture to a gentle simmer, whisking frequently. Let it cook for 5-7 minutes until it thickens noticeably and coats the back of a spoon. Reduce heat to low.

Step 4: Enrich and Season the Sauce
Remove the saucepan from the heat. Whisk in the Dijon mustard and Worcestershire sauce. Stir in the grated Gruyère and ½ cup of the Parmesan cheese until completely melted and smooth. Stir in the fresh herbs (parsley, tarragon), smoked paprika, cayenne (if using), lemon juice, and lemon zest. Taste the sauce critically—it should be highly seasoned, as it will be diluted by the seafood. Adjust with salt and white pepper as needed. The sauce should be thick, creamy, and luscious.

Step 5: Combine Seafood and Sauce
Gently fold the cooked seafood medley (scallops, shrimp, fish, and crab/lobster) into the cream sauce. Be careful not to break up the chunks of fish or crab. Pour the entire mixture into your prepared, buttered baking dish, spreading it into an even layer.

Step 6: Prepare and Apply the Crust
Preheat your oven to 375°F (190°C). In a medium bowl, combine the Panko breadcrumbs, the remaining ½ cup of Parmesan cheese, melted butter, chopped parsley, lemon zest, and minced garlic. Mix thoroughly until the crumbs are evenly coated and moist. Sprinkle this topping mixture evenly over the entire surface of the seafood and sauce, pressing down lightly.

Step 7: Bake to Golden-Brown Perfection
Place the baking dish on a rimmed baking sheet (to catch any potential bubble-overs) and bake in the preheated oven for 25-30 minutes, or until the sauce is bubbly at the edges and the topping is a deep, golden brown and crisp. If the top is browning too quickly, tent it loosely with aluminum foil. For a final perfect finish, broil on high for 1-2 minutes, watching constantly to achieve an ideal, even color.

Step 8: Rest, Garnish, and Serve
Remove the bake from the oven and let it rest on a rack for 10-15 minutes before serving. This rest is essential for the sauce to set slightly, making it easier to portion beautifully. Garnish with additional fresh parsley and a sprinkle of lemon zest. Serve directly from the baking dish with a large spoon or server, ensuring each portion gets a generous amount of the crispy topping and creamy seafood beneath.