01 - Preheat oven to 400°F. Line a baking tray with parchment paper.
02 - Toss butternut squash cubes with olive oil, salt, and pepper. Spread evenly on the tray and roast for 20 to 25 minutes until fork-tender and caramelized. Let cool slightly.
03 - Meanwhile, boil salted water and cook pasta according to package instructions until al dente. Drain and set aside.
04 - Puree the roasted butternut squash in a blender or food processor with a splash of milk until smooth.
05 - In a large saucepan, melt butter over medium heat. Add flour and whisk for 1 to 2 minutes to form a roux without browning.
06 - Gradually whisk in the milk, continuing to whisk until the sauce is smooth and thickened, about 5 minutes.
07 - Stir in the butternut squash puree, grated cheeses, nutmeg, and mustard powder. Cook until cheeses melt and sauce is smooth. Season with salt and pepper to taste.
08 - Add the cooked pasta to the sauce and stir until evenly coated.
09 - Transfer the mixture into a greased baking dish. In a small bowl, combine panko breadcrumbs, melted butter, and parmesan cheese. Sprinkle evenly over the pasta.
10 - Bake at 400°F for 15 minutes until topping is golden and bubbling. Let cool slightly before serving.