In a pan, over medium-high heat, add butter and onions and cook, stirring often, until onions are light pink. Add carrots and ginger. Cover and bring to a boil. Reduce heat and simmer until carrots are tender when pierced.
Remove from heat and allow to get cold, transfer to a blender. Don't fill the blender more than halfway, do it in batches.
Pulse the blender to start it and then puree until smooth. Sieve the mixture.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.
Serve hot by adding lemon juice.
