Preheat oven to 400. In a large pot of boiling salted water, cook macaroni 2 minutes less than al dente. Drain; rinse with cold water. Set aside.
While pasta is cooking, melt butter over medium in a large saucepan. Transfer 2 tablespoons melted butter to a medium bowl, and reserve. Add onion and garlic to pan; cook until softened, 3 to 5 minutes. Add flour; cook, stirring, 1 minute. Add milk; whisk until smooth. Bring to a boil; reduce to a simmer, and cook until sauce has thickened, 2 to 3 minutes. Season with 3/4 teaspoon salt, 1/2 teaspoon pepper, and nutmeg.
Remove pan from heat. Fold in seafood, macaroni, and cheese; transfer to a shallow 4-quart baking dish. To bowl with reserved melted butter, add breadcrumbs and Creole seasoning; toss, and scatter over macaroni mixture. Place baking dish on a rimmed baking sheet. Bake until topping is golden and sauce is bubbling, 15 to 20 minutes. Let cool 5 minutes before serving.
A touch of Creole seasoning (a blend of spices like garlic powder, paprika, and thyme) adds oomph. Store-bough