For the cream sauce:
4 tablespoons unsalted butter
1/4 cup all-purpose flour
1 (8-ounce) bottle clam juice
3 cups whole milk
1 1/2 teaspoons kosher salt
1/4 teaspoon freshly ground black pepper
For the lasagna:
1 pound dried lasagna noodles
1/2 bunch fresh parsley
1/2 medium lemon
4 cloves garlic
4 tablespoons unsalted butter
1/2 cup dry white wine, such as Sauvignon Blanc
1 pound tiny raw, peeled, and cleaned shrimp (70 to 90 per pound)
1 1/4 teaspoons kosher salt, divided, plus more for the pasta
3 cups part-skim ricotta cheese
1 1/2 ounces Parmesan cheese, finely grated (about 3/4 cup or scant 1/2 cup store-bought grated)
1/4 teaspoon freshly ground black pepper, plus more as needed
8 ounces lump crab meat
8 ounces cooked and chopped lobster meat
1 pound low-moisture mozzarella cheese, shredded (about 4 cups)
Arrange a rack in the middle of the oven and heat the oven to 350°F.
Make the cream sauce:
Melt 4 tablespoons unsalted butter in a medium saucepan over medium heat. Add 1/4 cup all-purpose flour and cook, stirring, until very lightly toasted, about 2 minutes. While whisking, slowly pour in 1 (8-ounce) bottle clam juice, then 3 cups whole milk. Season with 1 1/2 teaspoons kosher salt and 1/4 teaspoon black pepper. Bring to a boil over medium-high heat, stirring frequently. Reduce the heat to medium and simmer until thickened, 8 to 10 minutes more. Taste and season with more kosher salt and black pepper as needed.
Meanwhile, cook 1 pound dried lasagna noodles in a large pot of salted boiling water according to package directions until al dente. Drain, rinse under cool water until cool, and drain well again. Arrange the noodles in an even layer on a large baking sheet, coating them with olive oil and overlapping them slightly if needed.
Pick the leaves from 1/2 bunch fresh parsley and finely chop until you have 1/4 cup for the sauce and 1 tablespoon more for garnish. Juice 1/2 medium lemon until you have 1 tablespoon juice. Finely chop 4 garlic cloves.
Melt 4 tablespoons unsalted butter in a large skillet over medium heat. Add the garlic and cook, stirring, until golden brown, about 3 minutes. Add 1/2 cup dry white wine and boil until the wine is almost evaporated, about 3 minutes. Add 1 pound raw tiny shrimp and cook, stirring occasionally, until the shrimp are pink and just cooked through, about 4 minutes.
Remove the skillet from the heat. Add the 1/4 cup parsley, the lemon juice, and 1/4 teaspoon of the kosher salt, and stir to combine. Taste and season with more kosher salt as needed.
Place 3 cups ricotta cheese, 3/4 cup finely grated Parmesan cheese, the remaining 1 teaspoon kosher salt, and 1/4 teaspoon black pepper in a medium bowl and stir to combine. Taste and season with more kosher salt and black pepper as needed.
Assemble the lasagna:
Spread 1/2 cup of the cream sauce in the bottom of a 9×13-inch inch baking dish. Arrange 1/3 of the noodles over the cream sauce, overlapping them as needed. Spread half the ricotta mixture over the noodles. Evenly arrange half the shrimp mixture, half the lobster, and half the crab over the ricotta. Sprinkle with 1 cup of the mozzarella.
Dollop and spread 1 cup of the cream sauce over the cheese. Repeat layering half the remaining noodles, the remaining ricotta mixture, and the remaining shrimp mixture, lobster, and crab. Sprinkle with 1 cup of the mozzarella cheese. Dollop and spread 1 cup of the cream sauce over the cheese. Top with the remaining lasagna noodles, spread with the remaining cream sauce, and sprinkle with the remaining 2 cups mozzarella.
Coat a large sheet of aluminum foil with cooking spray. Loosely cover the baking dish the foil spray-side down. Bake for 30 minutes. Uncover and bake until golden-brown and bubbly, 20 to 25 minutes more. Let cool for at least 15 minutes before slicing. Garnish with the reserved parsley before serving.
Storage: Leftovers can be refrigerated in an airtight container for up to 2 days.