Saute the onion gently in the butter in a large sauce pan, covered, for five minutes, without browning. Add the carrots, ginger and a sprinkling of salt. Cover and cook for another 10 minutes. Stir occasionally and do not allow vegetables to brown. Add the stock or water and bring to a boil, then simmer gently for about 15 minutes, until the carrots are tender.Using an immersion blender, food processor, or traditional blender, puree the soup. Return the soup to the pan, reheat gently and season to taste with salt and pepper.