Preheat oven to 400 deg F. Cut the cauliflower into florets and place them with the carrots into a large baking dish (or lipped cookie tray) sprayed with oil. Lightly spray the top of the cauliflower with oil. Bake for 20 minutes. Sprinkle the onion wedges and garlic on top of the cauliflower, spray lightly with olive oil, and return to the oven. Cook for 20-25 more minutes, stirring once halfway through. While the vegetables are roasting, heat the vegetable broth and add the chopped potato. Bring to a boil and reduce the heat. Cook covered on very low until cauliflower is ready. Add the cauliflower mixture to the broth. Puree it, in batches, in the blender until very smooth. Return it to the pot and add the nutmeg and salt and pepper to taste. Simmer for 10 minutes. Just before serving, stir in a few drops of truffle oil, if desired. Ladle into bowls and serve, sprinkled with smoked Spanish paprika.