Add the cauliflower to a medium pot and cover with water. Place on the burner on medium-low heat and bring to a low simmer. Cover and continue to simmer for about 8-10 minutes or until cauliflower is soft. Strain the cauliflower and set it aside to allow it to cool slightly.
Using the same pot, heat the oil on low heat and saute the garlic and onion until fragrant. Set aside.
Add the cauliflower to a high-speed blender with garlic, onion, coconut milk, broth, lemon juice, nutritional yeast, salt, and pepper. Set aside.
Using the same large pan, add the spinach and cook for 2-3 minutes or until wilted. Add the cooked penne and alfredo sauce to the pan and stir to combine and coat the penne in the sauce.