In a 6-quart pan, over medium high heat, add butter and onions and cook, stirring often, until onions are limp. Add broth, carrots, and ginger. Cover and bring to a boil. Reduce heat and simmer until carrots are tender when pierced.
Remove from heat and transfer to a blender. Don't fill the blender more than half way, do it in batches if you have to. Cover the blender and then hold a kitchen towel over the top of the blender*. Be careful when blending hot liquids as the mixture can spurt out of the blender. Pulse the blender to start it and then puree until smooth. Return to the pan and add cream, stir over high heat until hot. For a smoother flavor bring soup to a boil, add salt and pepper, to taste.
Ladle into bowls and garnish with dollop sour cream and parsley sprigs.
*When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.
Go to full version of