In a large stockpot, heat olive oil over medium-high heat. Add onion, celery, carrots, and garlic and cook for 5 minutes, or until onion is translucent. Add spinach and cook for approximately 4 minutes to wilt. Add lentils, chicken broth, lemon zest, lemon juice, salt, and pepper. Increase heat to high. Bring soup to a boil. Serve immediately.