Bring a large pot of salted water to a boil. Add the cauliflower and boil until the cauliflower is fork tender, 8 to 10 minutes. Drain and place the cauliflower in a blender. Add the Parmesan cheese, olive oil, garlic, mustard, butter, lemon juice, salt, and several grinds of pepper.
Add 1 cup of the pasta water to the blender and blend the sauce until creamy.
As you toss your alfredo sauce with pasta, you may want to add the additional 1/2 cup pasta cooking water to loosen the sauce.