Cook lasagna noodles in a large pot of boiling salted water*
(see footnote below about salting pasta water)
until al dente. Drain well.
Preheat oven to 350°F. Lightly grease an 8-inch square pan.
In a medium-sized saucepan over medium heat melt butter, stir in flour to form a roux, and let flour brown slightly. Gradually whisk in milk, stirring constantly until sauce thickens. Add the Parmesan cheese to the sauce and stir well.
Spread a thin layer of white sauce into prepared pan, followed by a layer of cooked lasagna noodles. Place 1/4 of the shrimp, 1/4 of the crabmeat and 1/2 cup mozzarella cheese on top of the lasagna noodles. Repeat three times, making four layers. Top the final layer with sauce and remaining mozzarella cheese.