In a large pot, combine leeks, potatoes, carrots, bay leaf, thyme, salt and pepper. Add vegetable broth and enough water to cover vegetables. Bring to a boil. Turn heat to simmer and cook until vegetables are soft.

In a separate pan, heat soy milk. When vegetables are soft, add milk to vegetable mixture.

Make a roux, or paste, by combining the oil and flour. Stir into hot soup, a tablespoon at a time, allowing several minutes of cooking between each addition until soup reaches desires thickness.

Remove bay leaf before serving.