In a thick-bottomed soup pot over medium-high heat, cook onion, carrots and celery in oil until onions are translucent.Stir in garlic and cook another 30 seconds.Stir in spices and salt.Add lentils, stock and water and bring to a boil.Reduce heat and simmer for 40 minutes.Stir in tomatoes and continue to simmer for an additional 5-10 minutes.Let cool slightly before serving.