Heat oven to 350°F. Cook noodles as directed on package.
Meanwhile, in 3-quart saucepan, melt butter over medium heat. Add onion and garlic; cook 2 to 3 minutes, stirring occasionally, until onion is crisp-tender. Stir in flour; cook and stir until bubbly. Gradually stir in half-and-half, broth, sherry, salt and pepper. Heat to boiling, stirring constantly. Boil and stir 1 minute. Remove from heat and set aside.
In medium bowl, mix egg, Parmesan cheese, ricotta cheese and 1/4 cup parsley; set aside.
Drain noodles. In ungreased 13x9-inch (3-quart) glass baking dish, spread 3/4 cup of the sauce. Top with 3 noodles. Spread half of the crabmeat and half of the shrimp over noodles; spread with 3/4 cup of the sauce. Sprinkle with 1 cup of the mozzarella cheese; top with 3 noodles. Spread ricotta mixture over noodles; spread with 3/4 cup of the sauce. Sprinkle with 1 cup of the mozzarella cheese; top with 3 noodles. Spread with remaining crabmeat, shrimp and sauce. Sprinkle with remaining 1 cup mozzarella cheese.
Bake 40 to 45 minutes or until cheese is light golden brown. Let stand 15 minutes before cutting. Sprinkle with 1 tablespoon parsley.