Steam or boil the cauliflower & garlic: Bring a large pot of water (or broth) to a boil. Break the florets into large chunks and boil the cauliflower and peeled garlic for about 5-8 minutes. Strain from pot (save the water!) and place in blender. Add 1 cup cooking liquid and blend until mostly smooth. Add 2 cups cheese and blend until smooth. Add mustard and salt and blend until combined. Adjust taste as desired.
Bring the liquid back to a boil and cook the pasta for about 7-8 minutes, or until al dente. Drain and set aside.
Heat a large skillet over medium-high heat. Remove the sausage from the casing and crumble. Brown in pan until cooked through. Add the spinach and cook just until wilted.
Bake the casserole: Add the pasta, sausage, and spinach to a large oven-safe dish and stir well. Add the cauliflower sauce and mix, then top with the remaining cheddar cheese. Bake on 350ºF for 10-15 minutes. Serve!
Notes
For gluten-free: use any gluten-free pasta you enjoy! I prefer red lentil fusilli.