Preheat the oven to 375°F. Prepare the lasagna sheets according to the package directions if required.
Blanch the shrimp for about 90 seconds to 3 minutes, then transfer to ice water. Drain and chop. Mix with the crawfish and crab; season with seafood seasoning.
Melt the butter in a large pan. Sauté the onion, bell peppers and celery until softened, then add the garlic.
Stir in the flour and cook briefly. Gradually whisk in the seafood stock, milk and heavy cream. Simmer until the sauce thickens, stirring frequently.
Stir some of the Parmesan into the sauce. Season to taste with Cajun seasoning, all-purpose seasoning, bouillon, smoked paprika, herbs, lemon juice, salt and pepper.
Spread sauce in the bottom of the baking pans. Layer lasagna sheets, seafood, sauce, mozzarella and Parmesan, repeating until used. Finish with sauce and cheese.
Bake for 30–35 minutes, or up to 45 minutes, until hot and bubbling and the seafood is cooked through. Let rest before slicing and garnish with parsley.