Cook the macaroni until it is al dente, rinse and drain well.
Add 2 tbsp of melted butter to the panko bread crumbs in a small bowl, mix well to combine and set aside.
Place the other 4 tbsp of butter into a saucepan over medium heat on the stove.
Add the onions to the melted butter, and cook until they begin to soften.
Add the garlic into the skillet, and saute another 30 seconds.
Place the flour in the skillet, and stir to combine, cooking for 1 minute.
Whisk the milk and heavy whipping cream into the mixture in the saucepan, and add in the Old Bay seasoning, nutmeg, paprika and salt and pepper to taste.
Continue to cook the mixture until it thickens.
Whisk the cheeses into the sauce mixture until completely melted.
Stir the seafood into the cheese mixture, and cook until heated through.
Pour the cheese mixture over the macaroni and stir to combine.
Layer the macaroni in a 9×13 baking dish, and top with the bread crumb mixture.
