Heat oil in large soup pot; add onions and celery and sauté until onion is translucent. Stir in garlic and cook a minute more. Stir in lentils, liquid, tomatoes, oregano and bay leaf. Bring to a boil; reduce heat and simmer, partially covered until the lentils are tender, 35-45 minutes. Add salt, spinach and rice or pasta and cook until all is tender. If the soup is too thick, add some additional broth. Just before serving stir in lemon juice or vinegar.