In a medium saucepan, bring the cauliflower and 3 cups water to a boil. Reduce the heat to medium-low, cover and cook until the cauliflower is very soft, 10 to 15 minutes. Remove from the heat, drain and set aside.
Meanwhile, bring a large pot of salted water to a boil and add the pasta. Cook until al dente according to the package directions. Just before pasta is set to be done, add the spinach and cook until just wilted, about 30 seconds to 1 minute. Drain in a colander and set aside.
In the empty pasta pot, melt the butter over medium heat and add the garlic. Cook until the garlic is soft but not burnt, about 2 minutes.
Add the cooked garlic, steamed cauliflower, milk, 1/2 teaspoon salt, 1/4 teaspoon pepper and 1/2 cup water to a blender and puree until creamy and smooth. Depending on the power of your blender, you may need to add more water.
Return the cauliflower sauce back to the saucepan over medium heat. Whisk in the shredded Cheddar and stir until melted. Season to taste with salt and pepper, then stir in the cooked noodles, spinach and Parmesan. Serve immediately.
To transform this into a baked dish, add 1/4 cup more milk to the cauliflower mixture. Place the pasta and sauce in a casserole dish. Top with 1/4 cup breadcrumbs mixed with 1/8 cup grated Parmesan, then bake for 25 minutes at 350 degrees F.