Preheat your oven to 375°F. Butter or spray a 9x13 baking dish, set aside.
Cook the pasta as directed for al dente. Drain and set aside.
Meanwhile, in a large skillet on medium heat, melt 4 tablespoons butter.
Add the onion and cook until translucent.
Next, sprinkle flour over the onion and whisk to create a roux (a paste).
Cook for 2 minutes, then gradually whisk in the milk.
Continue cooking while whisking until slightly thickened, about 3 minutes.
Add the salt, mustard, pepper, and nutmeg, whisk to combine.
Reduce heat to low, stir in the mashed butternut squash.
Once warmed, add the cheese by the handful, whisking constantly. Continue stirring until smooth. If it becomes too thick, add a splash of milk.
Stir in the cooked pasta. Adjust seasonings as needed.
Pour the pasta and sauce into your baking dish.
Melt the remaining 1 tablespoon of butter, add the panko, and toss to coat. Sprinkle this over the pasta. Bake for 20 minutes or until golden and bubbly.
Serve this with a green salad and a big mound of buttered steamed broccoli. Carnivores could add crumbled bacon or diced ham to this recipe.