01 - Preheat oven to 400°F (200°C). Lightly grease a 2-quart baking dish.
02 - Cook the elbow macaroni in salted boiling water according to package directions until al dente. Drain thoroughly and set aside.
03 - In a large saucepan, melt 2 tablespoons of unsalted butter over medium heat. Sprinkle in the all-purpose flour and whisk continuously for 2 minutes to form a smooth roux.
04 - Gradually pour in the whole milk while whisking constantly. Continue to cook and whisk until the sauce is smooth and has thickened, approximately 4–5 minutes.
05 - Reduce heat to low and stir in the shredded cheddar, mozzarella, and grated Parmesan cheeses until fully melted and the sauce is creamy.
06 - Whisk in the Japanese curry powder, soy sauce, Dijon mustard, garlic powder, onion powder, salt, and black pepper. Taste and adjust seasonings as necessary.
07 - Gently fold the drained elbow macaroni into the cheese sauce, ensuring each piece is well coated. Transfer the macaroni and cheese mixture into the prepared baking dish.
08 - In a small bowl, combine the panko breadcrumbs, melted butter, curry powder, and grated Parmesan cheese. Mix until evenly moistened.
09 - Sprinkle the panko mixture evenly over the macaroni and cheese in the baking dish. Bake for 20–25 minutes, or until the topping is golden brown and the sauce is bubbling.
10 - Allow the Katsu Curry Mac and Cheese to rest for 5 minutes before serving. Garnish with chopped fresh parsley and sliced green onions, if desired.