Step 1: Prepare and Poach the Seafood
Pat the scallops and shrimp completely dry with paper towels. Season lightly with salt and white pepper. In a large skillet over medium-high heat, add the olive oil. Quickly sear the scallops for just 45-60 seconds per side to get a touch of color; they should still be translucent in the center. Remove to a plate. Add the white wine to the skillet, bring to a simmer, and add the shrimp. Poach the shrimp for 2-3 minutes, just until they turn pink and opaque. Remove shrimp with a slotted spoon to the plate with the scallops. Reserve the wine-poaching liquid—this is flavor gold.

Step 2: Build the Aromatic Base for the Sauce
In a medium saucepan over medium heat, melt the 4 tablespoons of butter. Add the minced shallots and cook until soft and translucent, about 4 minutes. Add the garlic and cook for 1 more minute until fragrant. Sprinkle the flour over the butter and shallots, whisking constantly to form a pale blond roux. Cook for 2 minutes to eliminate the raw flour taste.

Step 3: Create the Velvety Sauce
Slowly, while whisking vigorously to prevent lumps, stream in the warmed milk, followed by the heavy cream and seafood stock. Whisk until completely smooth. Pour in the reserved wine-poaching liquid from the seafood. Bring the mixture to a gentle simmer, whisking frequently, until it thickens enough to coat the back of a spoon, about 5-7 minutes. Reduce heat to low.

Step 4: Enrich and Season the Sauce
Remove the sauce from the heat. Stir in the grated Gruyère and ¼ cup of the Parmesan cheese until fully melted and incorporated. Add the chopped parsley, tarragon, smoked paprika (if using), nutmeg, and lemon juice. Taste the sauce—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 but still pourable.

Step 5: Assemble the Gratin
Preheat your oven to 375°F (190°C). Lightly butter your baking dish. Pour about one-third of the sauce into the bottom of the dish. Arrange the partially cooked scallops and shrimp evenly over the sauce. Pour the remaining sauce over the seafood, gently nudging it to ensure everything is covered.

Step 6: Create and Apply the Crumb Topping
In a small bowl, combine the Panko breadcrumbs, melted butter, olive oil, chopped parsley, lemon zest, and the remaining ¼ cup of Parmesan cheese. Mix until the crumbs are evenly moistened. Sprinkle this mixture evenly and generously over the entire surface of the sauced seafood.

Step 7: Bake to Golden-Brown Perfection
Place the dish on a baking sheet (to catch any potential bubble-overs) and bake in the preheated oven for 20-25 minutes, or until the sauce is bubbly around the edges and the topping is a deep, uniform golden brown. If the top is browning too quickly, you can tent it loosely with foil. For a final blast of color and crispness, broil for 1-2 minutes at the end, watching constantly to prevent burning.

Step 8: Rest and Serve
Remove the gratin from the oven and let it rest for 8-10 minutes before serving. This allows the sauce to settle and thicken slightly, making serving easier. Garnish with additional fresh herbs and a lemon wedge on the side. Serve directly from the baking dish with a large spoon, ensuring each portion gets plenty of seafood, sauce, and that glorious crunchy topping.