Cook the pasta as directed. Drain the cooked pasta, pour into a large bowl and stir in 1 Tablespoon of butter. Set aside.
In a large sauce pan, melt the remaining 4 Tablespoons of butter over medium heat with the onion and garlic. Sauté until the onions are very soft, about 5-7 minutes. Once cooked, whisk in the flour to form a thick paste. Cook and whisk for 1 minute, then whisk in the milk, then pumpkin puree. Bring to a simmer. Then, whisk in the shredded cheese. Stir in the ground mustard, dried parsley, salt, and pepper.
Serve immediately. Garnish with fresh parsley, if desired.
Leftovers stay fresh in the refrigerator for up to 5 days. It can get thick in the refrigerator as the pasta soaks up the sauce, but warming them up in the microwave thins out the sauce.
Notes
Make Ahead Instructions: This can be made in advance up to 1 day. Store in the refrigerator, covered with foil, bake for 30 minutes until heated through.
Pasta: Any small-type pasta such as rotini, elbows, or shells works best for this recipe.
Milk: For best results, use whole milk.
Cheese: Any type of cheese works for this recipe, though we love to use sharp cheddar.