Preheat your oven to 375°F (190°C). Cook the lasagna noodles according to package instructions, then drain and set aside on a lightly greased baking sheet to prevent sticking.

In a medium saucepan, melt butter over medium heat. Add the flour and whisk constantly for 1–2 minutes until it forms a roux. Gradually add milk, whisking continuously to prevent lumps. Cook until thickened, about 5–7 minutes. Stir in garlic, onion powder, salt, pepper, and optional nutmeg. Remove from heat and set aside.

In a large mixing bowl, combine ricotta cheese, chopped parsley, lemon juice, and half of the Parmesan cheese. Mix until smooth. Fold in the chopped shrimp and lump crab meat gently, keeping the chunks intact.

Spread a thin layer of béchamel sauce on the bottom of your baking dish. Place a layer of cooked lasagna noodles on top. Spread one-third of the seafood-ricotta mixture over the noodles, then drizzle a third of the béchamel over the top. Sprinkle a portion of mozzarella cheese. Repeat layers two more times, finishing with béchamel and remaining mozzarella and Parmesan cheese on top.

Cover the lasagna with foil and bake for 30 minutes. Remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.

Remove from the oven and let rest for 10 minutes before slicing. Garnish with chopped fresh parsley for color and freshness. Serve warm.