Bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions until al dente.
Heat the olive oil in a large skillet over medium low heat. Add the onions and sauté until translucent, then add the butternut squash, minced garlic, mustard powder and cayenne pepper, and cook for a few more minutes.
Add the vegetable broth and milk to the skillet, bring to a boil and cook until the butternut squash is fork tender, about 10 minutes.
Blend all the ingredients using a hand immersion blender until sauce is smooth and creamy. Alternatively, you can transfer the ingredient to a blender and blend.
Fold the cooked pasta and shredded cheese into the pureed sauce and stir to combine. Stir in extra milk if you'd like to adjust the consistency.
Sprinkle with chopped parsley, if desired and serve warm.