Heat the margarine or water in a large saucepan. Cook the onion for 5 minutes over low heat, or until soft. Add the carrots and potato, cover, and simmer for 5 to 10 minutes. Add the broth, cover, and simmer about 20 minutes, or until vegetables are tender.
Pour the soup through a sieve or colander, reserving the stock for later. Puree the vegetables in a blender or food processor and return them to the saucepan. Add the soy milk, then gradually add the reserved stock until the soup reaches desired consistency.
Flavor with the remaining ingredients. Reheat slowly and serve.