01 - Set the oven to 400°F and lightly grease a large baking dish.
02 - Boil pasta in salted water until al dente, then drain and set aside.
03 - Heat 1 tablespoon butter in a large skillet over medium heat. Sauté the onion for 2 minutes until translucent, then add garlic and shiitake mushrooms, cooking an additional 2 minutes.
04 - Add shrimp, squid, and fish to the skillet and cook until just opaque, about 2–3 minutes. Stir in spinach and cook until wilted. Transfer mixture to a bowl.
05 - Melt 2 tablespoons butter in the same skillet over medium heat. Whisk in flour and cook for 1 minute. Gradually add milk, whisking until smooth and thickened, roughly 4–5 minutes.
06 - Stir in Japanese mayonnaise, soy sauce, mirin, and dashi powder (if using). Adjust salt and white pepper to your preference.
07 - Return seafood and vegetable mixture to skillet, add cooked pasta, and fold gently until evenly coated.
08 - Pour the combined mixture into the prepared baking dish.
09 - Mix panko breadcrumbs, Parmesan cheese, and melted butter in a small bowl. Sprinkle this mixture evenly over the top.
10 - Bake at 400°F for 20–25 minutes, until the topping is golden and the dish bubbles.
11 - Optionally, garnish with chopped parsley and let cool for 5 minutes before serving.