Heat oil in a dutch oven over medium-low heat. Add the onion and garlic. Saute for 5 minutes or until onion begins to get tender.
Add the stock and remaining ingredients except the kielbasa. Simmer for approximately 1 hour or until lentils are tender. Remove thyme sprigs and bay leaves.
Puree soup either in a conventional blender or with an immersion/stick blender.
Add the kielbasa and heat through.
Notes
Omit the kielbasa and use vegetable stock for a vegan soup.