Bring a large pot of water to a boil. Season generously with salt. Add the pasta and cook for 3 minutes less than the package instructions. Drain and set aside.
Meanwhile, heat a large pan over medium heat and add the oil. When hot, add the cauliflower and onions and saute until starting to soften, 3 to 5 minutes. Add the garlic and cook for another minute. Add the water or stock, bring to a simmer and simmer until everything is soft, about 3 minutes. Add the nutmeg. Check the seasoning and add salt and pepper to taste. Remove from the heat and let cool slightly. Transfer to a blender and puree until smooth.
Toss the pasta with the puree in a medium bowl. Transfer to a baking dish and top with the breadcrumbs, herbs and dairy-free Parmesan. Bake until warmed through and the breadcrumbs are toasted, 3 to 5 minutes.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.