Step 1: Prepare the Seafood Mixture

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and garlic and sauté until softened and fragrant, about 2-3 minutes. Add the shrimp and cook until pink and opaque, about 4-5 minutes. Stir in the crab meat and spinach (if using), and season with salt and pepper. If you want extra flavor, deglaze the skillet with white wine and allow it to cook off. Remove from heat and set aside.

Step 2: Prepare the Bechamel Sauce

In a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for about 2 minutes to create a roux. Slowly add the warm milk, whisking constantly to prevent lumps. Continue to cook and whisk until the sauce thickens, about 5-7 minutes. Stir in garlic powder, nutmeg, and salt and pepper to taste. Once thickened, remove from heat and stir in 1/2 cup of Parmesan cheese.

Step 3: Assemble the Lasagna

Preheat your oven to 375°F (190°C). In a 9×13-inch baking dish, spread a small amount of the white sauce at the bottom. Layer lasagna noodles on top, followed by a portion of the seafood mixture, some ricotta cheese, and a drizzle of white sauce. Repeat the layers until all ingredients are used up, finishing with a layer of noodles, sauce, and a generous amount of mozzarella and Parmesan cheese on top.

Step 4: Bake the Lasagna

Cover the lasagna with aluminum foil and bake in the preheated oven for 25-30 minutes. After that, remove the foil and bake for an additional 10-15 minutes or until the top is golden and bubbly.

Step 5: Let It Rest and Serve

Once baked, remove the lasagna from the oven and let it rest for about 10-15 minutes before slicing. Garnish with fresh parsley and serve warm.