Melt the butter in a medium saucepan over medium heat. Add the onion and sauté until starting to soften but not yet turn color, about 5 minutes, stirring occasionally.
Add the sweet potato and cook 3 minutes more. Add the salt, pepper, nutmeg, bay leaf, stock, pumpkin, and brown sugar.
Bring the soup up to a boil, then turn heat down and cook until the sweet potato is tender, about 15 to 20 minutes, stirring occasionally.
Turn heat off and stir in the cream, and then the lemon juice. Taste and season with additional salt and pepper as desired.
If you want a very smooth soup, puree it in batches in a blender (or use an immersion blender).
Ladle the soup into bowls and sprinkle the parsley on top, if desired.