Boil a large pot of salted water over high heat. Once boiling, add the cauliflower florets and cook for about 8-10 minutes or until fork-tender. Drain and transfer to a blender, letting cool slightly.

In the blender with the cauliflower, add Parmesan cheese, olive oil, garlic, mustard, butter, lemon juice, salt, and black pepper. Blend until smooth and creamy.

In another large pot, bring salted water to a boil. Add fettuccine and cook according to package instructions (about 8-10 minutes) until al dente. Reserve 1.5 cups of pasta cooking water, then drain.

In a large bowl, combine the drained fettuccine with the creamy cauliflower sauce. Gradually add the reserved pasta water until desired consistency is reached.

Garnish with freshly chopped parsley and adjust seasoning as needed. Serve immediately with extra Parmesan cheese.