Combine the oil and garlic in a medium heavy-bottomed saucepan and heat over medium heat, swirling, until the garlic is soft and fragrant but not brown, about 3 minutes. Add the cauliflower, nutmeg, 2 3/4 cups water and 1 teaspoon each salt and pepper. Cover and cook until the cauliflower is completely soft, 15 to 20 minutes. Add the yeast or Parmesan and puree with an immersion blender until the sauce is very smooth.
Meanwhile, bring a large pot of salted water to a boil and add the fettucine. Cook according to the package directions. Reserve at least 1/4 cup of the pasta water, then drain the pasta and add to the cauliflower sauce. Toss to coat the pasta and heat over medium low until the sauce is warmed through. Add some of the reserved pasta water to adjust the consistency if needed. Season with salt and pepper.
Divide among 4 serving bowls and sprinkle with parsley, nutritional yeast or Parmesan and crushed red pepper if using.
Copyright 2017 Television Food Network, G.P. All rights reserved.