Melt the butter in a saucepan over a medium heat. Add carrots and onion and cook and stir for about 7 to 10 minutes, until tender. Stir in the curry powder and ginger, cooking until fragrant, about 2 minutes.
Blend the soup using a food processor or handheld liquidiser until smooth. Return to the heat and stir in lemon zest, salt and pepper.
Pour in chicken stock and bring to the boil. Reduce to a simmer and cook until vegetables are tender, about 20 minutes. Remove from the heat, ladle into warm bowls and serve.