Melt one-third of the butter in a heavy-based pan and sauté the diced onion in it, stirring until it becomes translucent.
Add another third of the butter and stir in the flour. Cook, stirring, over a low heat for 2 minutes.
Whisk in the cider and bring quickly to a boil. Lower to a simmer, season with salt, pepper and grated nutmeg, and cook for 10 minutes.
Add the crème