Cut the cauliflower into large pieces and place in a large pot with enough water to cover the cauliflower. Cover and bring to a boil over high heat. Reduce the heat and simmer for 5 to 8 minutes or until the cauliflower is fork tender.
Meanwhile, cook the pasta according to package directions. Before straining, retain a cup of pasta water in case the sauce is too thick.
Add the cooked cauliflower, baby spinach, milk, cheese, salt, and optional pepper to a blender or food processor. Blend for 1 to 2 minutes until the sauce is creamy and smooth. Add ¼ cup of pasta water at a time to loosen the sauce if it is too thick.
Combine either the full amount of sauce with the pasta if you want a saucy pasta or half the sauce with the pasta (put the remaining sauce in the fridge or freezer) if you want a boxed mac and cheese like consistency.
Top with additional salt or pepper if desired.