Preheat the oven to 375ºF (190ºC).
Halve the pumpkin from top to bottom (remove the seeds and discard) and place it, cut side down, on an oiled baking sheet. Bake until the pumpkin is soft for about 45 to 60 minutes. Cool for about 20 minutes.
With a spoon, scrape the pulp and discard. Melt butter in a pot over medium heat. Add the onions and apple, and cook, stirring occasionally until the onions and apples are soft. Add the pumpkin and water, and simmer for about 30 minutes. Let cool.
In batches, purée the soup in a blender on high speed, about 3 minutes per batch, until very smooth. Strain through a fine mesh into a pot and add the cream. Bring to a boil. Adjust seasoning with salt, pepper and nutmeg.
Whip ¼ cup cream to soft peaks and add cinnamon. Ladle the hot soup into bowls. Add 1 spoon of cinnamon cream.