Preheat oven to 375°F (190°C). Butter an 8x8 inch baking dish. Ensure all seafood is patted very dry.
In a medium saucepan over medium heat, melt 4 tablespoons of butter. Whisk in the flour and cook for 1 minute, stirring constantly.
Slowly whisk in the whole milk, followed by the heavy cream. Add the sherry, onion powder, garlic powder, salt, and pepper.
Bring the sauce to a gentle simmer and cook, stirring often, until it thickens enough to coat the back of a spoon (about 5-7 minutes). Remove from heat.
Gently fold the haddock, shrimp, and scallops into the cream sauce.
Pour the mixture into the prepared baking dish.
In a small bowl, combine the crushed Ritz crackers with the remaining 4 tablespoons of melted butter. Sprinkle evenly over the casserole.
Bake for 20-25 minutes, until the sauce is bubbly and the topping is golden brown.
Let the casserole rest for 5-10 minutes before garnishing with fresh parsley and serving.
Notes
Make sure your seafood is patted very dry with paper towels before adding it to the sauce. This is the key to preventing a watery casserole.
Keyword Coastal New England Seafood casserole, new england seafood bake, seafood casserole