01 - Preheat oven to 400°F and line a baking sheet with parchment paper. Toss diced butternut squash with 1 tablespoon olive oil, a pinch of salt and pepper. Arrange in a single layer and roast for 20 to 25 minutes until tender and lightly browned.
02 - While the squash roasts, bring a large pot of salted water to a boil. Add pasta and cook until al dente per package directions. Drain well and set aside.
03 - In a large saucepan over medium heat, melt 2 tablespoons unsalted butter. Add diced onion and cook for 3 to 4 minutes until softened. Stir in minced garlic and cook for 1 additional minute.
04 - Add roasted butternut squash to the saucepan with onion and garlic. Pour in 1 ½ cups whole milk. Puree the mixture with an immersion blender (or transfer to a blender) until completely smooth.
05 - Return the pureed mixture to the saucepan. Stir in sharp cheddar, Gruyère, Parmesan, and sour cream. Mix in mustard powder and nutmeg. Cook, stirring, until cheese is melted and sauce is silky. Adjust seasoning with additional salt and pepper to taste.
06 - Gently fold the cooked pasta into the cheese sauce, ensuring even coating.
07 - For a baked topping, preheat broiler. Mix panko breadcrumbs with melted butter, then sprinkle over pasta in an oven-safe dish. Broil for 2 to 3 minutes until golden brown.
08 - Dish out hot, garnished with extra Parmesan or fresh chopped herbs if desired.