Cook the macaroni noodles a little ahead of time cooking 2 minutes less than al-dente using the package as your guide. Drain and rinse with cold water to stop further cooking.
If the scallops are full sized, slice them horizontally to create 1/4 to 1/3 inch slices. Pat dry on paper towel. Heat a large non-stick skillet to medium high and add a tablespoon of butter. Sear the scallops turning only once so you have a nice golden brown on each side but still soft. Remove scallops from pan and set aside.
Preheat the oven to 400ºF
Cut the peeled and deveined shrimp into good bite sized pieces. Add another tablespoon of butter to the skillet and cook the shrimp for about 3 minutes, until no longer pink. Remove from the skillet and set aside.
In a large saucepan, melt the remaining 6 tablespoons of butter. Spoon out 2 tablespoons of the butter and mix it into the panko breadcrumbs.
Add the diced onion and minced garlic to the saucepan and cook for 3 to 4 minutes, until the onion has softened stirring continuously. Add in the flour, continuing to stir for about 1 minute.
With a whisk, slowly add the milk, whisking to make sure the mixture is smooth and free of lumps. Bring to a boil then reduce the heat to a simmer, stirring until it has thickened.
Add in the salt, pepper, nutmeg and Old Bay Seasoning.
Remove the saucepan from the heat and stir in the three cheeses, stirring until fully melted and smooth. Fold in the macaroni noodles and all of the seafood.
Pour the mixture into a buttered oven proof casserole large enough to hold it all or use 2 smaller prepared casseroles if it is not all going to be used immediately.
Spread the buttered panko breadcrumb mixture over the top and bake for about 20 minutes or until the top is nice and golden.
Garnish with fresh parsley and serve.
