Heat oil in stock pot over medium-low heat. Sauté onion, carrot, and celery for 8-10 minutes, until vegetables are soft and onion is translucent.
Add water, tomato juice, basil, and lentils to pot. Bring to boil, then turn down heat, cover, and simmer for 45 minutes.
Add potatoes, kosher salt and pepper. Cover, and continue to simmer for an additional 45 minutes, stirring frequently. If soup has cooked down and is too thick, add water to desired consistency.
Add spinach, and cook, uncovered for 20 minutes. Serve and enjoy!