Heat the oil in a large wide pot over medium heat. Add the onion, a large pinch of salt and a few grinds of pepper; cook, stirring occasionally, until the onions are tender but not browned, about 10 minutes. Add the garlic, rosemary and nutmeg; cook until the garlic is softened, about 30 seconds more.
Add the parsnips, cauliflower, broth and 2 cups water. Bring to a boil, reduce the heat to maintain a simmer, cover and simmer until the parsnips and cauliflower are tender, about 20 minutes. Cool 5 minutes.
Carefully puree in batches in a blender until smooth, taking care not to fill the carafe more than half full per batch, or directly in the pot with an immersion blender. Reheat the soup over medium heat, stirring occasionally, until steaming. Stir in the half-and-half and adjust the seasoning with additional salt and pepper. Add a few splashes of hot water if the soup is too thick. Top with a piece of roasted cauliflower and roasted parsnip. Sprinkle with the chives.
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