Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: In a large skillet, heat olive oil over medium heat. Add minced garlic and cook for 1 minute until fragrant. Add shrimp and cook for 2-3 minutes, just until they turn pink. Remove shrimp from heat and roughly chop if desired. Gently fold in the drained crab meat. Set aside.
Step 3: In a medium bowl, combine ricotta cheese, egg, chopped fresh parsley, Italian seasoning, salt, and black pepper. Mix until well combined.
Step 4: Spread about 1 cup of marinara sauce on the bottom of the prepared baking dish.
Step 5: Arrange 3-4 lasagna noodles over the sauce, slightly overlapping if necessary. Spread half of the ricotta mixture evenly over the noodles.
Step 6: Top the ricotta layer with half of the shrimp and crab mixture, then sprinkle with 1 cup of mozzarella cheese and 1/4 cup of Parmesan cheese. Spoon another 1 cup of marinara sauce over the cheese.
Step 7: Repeat the layering: noodles, remaining ricotta, remaining shrimp and crab, 1 cup mozzarella, 1/4 cup Parmesan, and another 1 cup of marinara sauce.
Step 8: Place the final layer of noodles, then top with the remaining marinara sauce, the last 1 cup of mozzarella cheese, and a sprinkle of Parmesan.
Step 9: Cover the baking dish tightly with aluminum foil. Bake for 25 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and lightly golden. If desired, you can broil for the last 2-3 minutes for extra browning.
Step 10: Let the lasagna rest for 10-15 minutes before slicing and serving. This allows the layers to set and makes for cleaner portions.