Step 1: If using traditional lasagna noodles, cook according to package directions until al dente. Drain and rinse with cold water to prevent sticking. Set aside.
Step 2: In a large skillet, heat olive oil over medium-high heat. Add shrimp and cook for 2-3 minutes per side, until pink and cooked through. Remove shrimp from the pan and set aside. Once cooled, chop shrimp into bite-sized pieces.
Step 3: In the same skillet (or a separate saucepan), melt butter over medium heat. Whisk in flour and cook for 1-2 minutes, stirring constantly, to create a roux.
Step 4: Gradually whisk in the warm milk and heavy cream, a little at a time, until smooth. Continue whisking until the sauce thickens and comes to a gentle simmer. Reduce heat to low.
Step 5: Stir in the grated Parmesan cheese, minced garlic, salt, pepper, and nutmeg. Cook for another 2-3 minutes, stirring, until the cheese is melted and the sauce is creamy. Remove from heat. This is your Alfredo sauce.
Step 6: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 7: Spread a thin layer of Alfredo sauce on the bottom of the prepared baking dish. Arrange a single layer of lasagna noodles over the sauce.
Step 8: Spread about one-third of the remaining Alfredo sauce over the noodles. Sprinkle with one-third of the chopped shrimp and crab meat. Top with one-third of the mozzarella cheese.
Step 9: Repeat Step 8 two more times, creating three layers of noodles, sauce, seafood, and cheese. For the top layer, ensure it's generously covered with sauce and mozzarella.
Step 10: Cover the baking dish loosely with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the top is golden brown and bubbly. If desired, you can broil for the last 2-3 minutes for extra browning, watching carefully.
Step 11: Remove from oven and let the lasagna rest for 10-15 minutes before slicing and serving. Garnish with fresh chopped parsley if using.