Preheat your oven to 350°F (175°C).
In a large skillet, melt butter and sauté onions and garlic until soft.
Add shrimp, scallops, and imitation crab meat. Cook until seafood is opaque, about 5 minutes. Set aside.
In the same skillet, sprinkle in the flour and cook for 1 minute.
Gradually whisk in the milk and cream. Stir constantly until thickened. Season with salt, pepper, and cayenne. Remove from heat.
In a small bowl, mix ricotta cheese with egg.
In a greased 9x13-inch baking dish, spread a thin layer of sauce. Layer 3 lasagna noodles, 1/3 of the ricotta mixture, 1/3 of the seafood, 1 cup mozzarella, and a layer of sauce. Repeat layers twice.
Top with remaining sauce, mozzarella, and Parmesan cheese.
Cover with foil and bake for 35 minutes. Remove foil and bake an additional 10-15 minutes until golden and bubbly.
Let it rest for 10 minutes before slicing. Garnish with parsley.