Use green lentils, or other firm lentils for this soup. Red lentils will work, but the measurements aren't the same. If you are looking for a great red lentil recipe, try this sweet potato red lentil curry.
In a large heavy-bottomed pot, warm the oil over medium high heat.
Add the onion, celery, and garlic and sauté until the onion is translucent, about 3 minutes.
Add the carrots, potato, lentils, and vegetable broth and bring to a simmer.
Reduce heat to medium-low, then add the fennel seeds, thyme, celery seeds, sage, and bay leaf.
Simmer for about 30 minutes, or until the lentils and vegetables are tender.
Season the soup with the soy sauce and white pepper.
Season to taste with salt––you'll need to add more if you used unsalted or low-sodium vegetable broth, and less if you used the Better Than Bouillon paste.