Sauté shrimp in butter and garlic over medium heat for 1–2 minutes just until pink. Do not fully cook — they will finish in the oven. Set aside. Flake crab meat and check for shell pieces.
In the same pan, melt butter over medium heat. Cook onion until softened. Stir in flour and cook for 1 minute. Slowly whisk in milk until smooth. Simmer until thick enough to coat a spoon. Remove from heat and stir in Parmesan, ricotta, salt, white pepper, nutmeg, and parsley. Fold in the shrimp and crab.
If using regular noodles, cook to al dente, drain, and lay flat on parchment. Skip if using no-boil.
Preheat oven to 375°F (190°C). Spread a thin layer of sauce in the bottom of a 9×13 inch baking dish. Layer noodles, then one-third of the sauce, then mozzarella. Repeat twice more. Finish with noodles, remaining sauce, and a generous top layer of mozzarella and Parmesan.
Cover with foil and bake for 30 minutes. Uncover and bake for a further 15–20 minutes until golden and bubbling. Rest for 15 minutes before slicing.