To make the soup, heat the olive oil in a large pot or Dutch over over medium heat. Add the onion garlic and cook until the onion is softened and translucent, 4 to 5 minutes. Add the cauliflower, 1 1/2 teaspoons salt and 3/4 teaspoon pepper and stir to coat. Stir in the apple cider and water. Bring to a boil, then reduce the heat and simmer until the cauliflower is very tender, about 25 minutes. Cool the soup slightly, then puree in batches in a blender or with an immersion blender (if you have a high-powered blender, like a Vitamix or Blendtec, use it! It makes the soup exceptionally silky). Return the soup to the pot and stir in the Parmesan.