In a large soup pot, sauté the onion in the butter or oil over medium heat for 5 minutes until golden. Add the leeks and carrots and continue cooking and stirring, until the leek is softened, about 8 minutes. Mix in ginger and cook until fragrant, 1 minute longer. Add water and salt and bring to a boil. Reduce heat, cover, and simmer soup until carrots are very tender, about 20 minutes. Let cool a few minutes.