Melt the butter in a saucepan over medium heat. Add the onions, sprinkle with a pinch of salt, and cook slowly until soft. Stir often.
Add the wine and bring to a boil. Cook for about a minute. Add the milk and stock and bring to a boil. Reduce to a simmer. Sprinkle the riceflour over the soup and stir well. Stir fairly often until the flour no longer settles to the bottom. Cook the onions for about 20 minutes. Add some pepper and nutmeg to taste.
Transfer the soup to the jar of a blender, add the cream, and puree.
Strain the puree into a clean saucepan and gently reheat. Taste for salt.