Preheat your oven to 375°F. Bring a large pot of salted water to a boil and cook the lasagna noodles until al dente, about 8 minutes. Drain and set aside.
Meanwhile, in a large sauté pan over medium heat, warm the olive oil. Add the onion, garlic, red pepper flakes, black pepper, and kosher salt and cook, stirring occasionally, for about 5 minutes.
Add the shrimp and scallops to the pan and cook for another 3 minutes. Add the crabmeat and cook for 1 minute more or until the shrimp is a beautiful pink while the scallops have a milky, almost translucent opalescence. Remove from heat and set aside.
In a medium bowl, mix together the ricotta cheese, Parmesan cheese, spinach, parsley, lemon zest, and nutmeg.
Spread the marinara sauce over the base of a 9 x 13-inch baking dish, then layer on 4 lasagna noodles. Dollop the ricotta mixture evenly over the noodles and spread it out with a spatula, followed by 1/2 of the seafood mix and 1/2 cup mozzarella cheese.
Repeat with the remaining noodles, ricotta cheese mixture, seafood mixture, and marinara sauce. Sprinkle the mozzarella cheese over the top.
To make the white sauce, melt the butter in a small saucepan over medium-low heat. Add the flour and whisk until smooth. Slowly add the milk, whisking constantly until the sauce is thick and creamy. Season with salt and pepper to taste.
Pour the white sauce over the lasagna. Cover with aluminum foil and bake for 30 minutes. Uncover and cook for an additional 15 minutes or until bubbly and golden brown. Let cool before serving.
Serve with extra Parmesan cheese, if desired.