In a large casserole pan (a 9 by 13 will make all but 2 of your manicotti noodles), lay out your manicotti noodles. Pour boiling water over them until they are covered. Let soak 30 minutes. Drain off and let the noodles cool.
Mix the seafood, egg, onion, mozzarella cheese, Old Bay seasoning, mayo, and the 2 tablespoons of milk together.
Stuff the seafood filling into the cooled manicotti. To make this easier, use a Ziplock bag with a corner cut off.
In a medium heavy saucepan, melt the butter on low/medium heat.
When the butter is melted add the cornstarch and whisk just until smooth.
Add the minced garlic and whisk gently for about 1 minute or until fragrant.
Slowly pour in the milk, whisking continuously. Heat until the sauce thickens up a bit. Do NOT boil.
Add the parmesan cheese, return the pan to the heat, and continue stirring until the cheese is melted and the sauce is smooth.
Add the salt & pepper. Taste and add additional salt & pepper, if needed.
Spread the ricotta over the stuffed manicotti and top with the sauce. NOTE: if you are making a 9 by 13 pan you most likely won't use about ½ cup of the sauce.
Put into 350 degree preheated oven and bake for 45 minutes or until hot and bubbly.
Remove from the oven, turn the oven to broil. Top with parsley and broil for about 5 minutes or until golden brown.
Let cool before serving. Serve with a squeeze of lemon and sprinkle of fresh parsley.