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.

Bring another large pot of salted water to a boil. Prepare the pasta according to the package instructions, cooking until al dente. Reserve 1 1/2 cups of the starchy pasta cooking water, then drain the pasta.

Add 1 cup of the pasta water to the blender and blend the sauce until creamy.

Return the pasta to the pot. Add the sauce and toss to coat. If the sauce is too thick, add the remaining 1/2 cup pasta water and toss again to loosen the sauce and make it a bit creamier.

Season to taste and serve topped with parsley and more cheese, if desired.