Preheat oven to 375 degrees F.
For the sauce: Melt the butter in a saucepan. Add minced shallots and saute for 1 to 2 minutes. Add the flour to make a roux. Cook for about 2 to 3 minutes to cook the flour. Slowly, add 3 cups milk. When milk is incorporated into the roux, add the seafood seasoning, and the block of cream cheese. Cook until cream cheese is melted. Add shredded Manchego and Cheddar cheeses and parsley. Cook until all the cheese is melted. Add the sherry, if desired.
Cook macaroni according to package directions or until just al dente. Drain macaroni. Stir cheese mixture into cooked macaroni and old in crabmeat. (Be sure to fold in crabmeat so you don't break up the lumps too much, you want nice chunks of crab.) Put mixture in a baking dish.
For the topping:
Mix melted butter, bread crumbs, seafood seasoning, and parsley. Sprinkle topping on the dish. Bake in the oven for 15 to 20 minutes to make the topping crispy. Garnish with fresh parsley.