Preheat the oven to 350°F.
Over medium heat, in a large skillet, melt the butter. Add the flour and whisk until smooth. Cook for 1-2 minutes, stirring constantly.
Slowly whisk in the heavy cream and chicken broth until smooth.
Add the Parmesan cheese, scallions, parsley, basil, and dill. Stir until the cheese is melted and the herbs are well incorporated.
Add the shrimp, scallops, and crabmeat to the skillet. Season with salt and black pepper to taste.
Transfer the seafood mixture to a 9x13 inch baking dish.
In a small bowl, mix the panko breadcrumbs with 2 tbsp. melted butter. Sprinkle the breadcrumb mixture over the seafood.
Bake the casserole for 25-30 minutes or until the top is golden brown and the seafood is cooked through.
Serve the casserole hot with crusty bread or rice.