In a Dutch oven or large soup pot heat oil over medium heat. Add the onions, carrots, and celery stirring occasionally until the onions become translucent, about 3 to 5 minutes. Add in garlic, bay leaf, oregano, and basil and cook until fragrant, about 2-3 minutes.
Add the broth, tomatoes, salt, pepper, and lentils. Bring to a boil and then reduce to a simmer. Cover and cook until the lentils are tender, at least 1 hour.
Just before serving add the spinach to the pot and the heat will cause the spinach to wilt.