Heat olive oil in a large pot over medium heat.
Add diced onion, and sauté for 4-5 minutes until translucent.
Stir in minced garlic, carrots, and celery; cook for another 3-4 minutes.
Add cumin, smoked paprika, and thyme; stir to coat the vegetables.
Pour in rinsed lentils, diced tomatoes (with juices), and vegetable broth.
Bring to a boil, then reduce heat and simmer uncovered for 30 minutes, or until lentils and vegetables are tender.
Add diced zucchini and cook for an additional 10 minutes.
Stir in fresh spinach and cook until wilted, about 2 minutes.
Season the soup with salt, black pepper, and lemon juice to taste.
Serve hot, garnished with fresh parsley.