Bring a large pot of boiling water to a boil. Add the cauliflower and cook, covered, for 8-10 minutes or until cauliflower is fork tender. Reserve 1 cup of the cooking water.

Heat the olive oil in a large skillet over medium heat, sauté the onion and garlic with crushed red pepper until the garlic is soft and fragrant, but not browned, about 1-2 minutes.

Use a slotted spoon to transfer the cooked cauliflower to the skillet. Season with salt and pepper and cook until fragrant, 1 minute.

Transfer the cauliflower mixture to a blender along with the reserved cooking liquid and milk. Cover the vent in your blender lid with a thin kitchen towel to allow venting and puree until the sauce is smooth and creamy, about 1 minute. If the sauce starts to look dry, add a few drops of water, milk, or olive oil.

Toss the alfredo sauce with cooked fettucine and serve immediately.