Chop onion, dice carrots and celery, and mince garlic.
Heat olive oil in a large pot; sauté onion, carrots, and celery for 5–7 minutes until softened. Add garlic, thyme, oregano, smoked paprika, and red pepper flakes; sauté for another minute.
Stir in lentils, vegetable broth, and diced tomatoes. Bring to a boil, reduce heat to low, cover, and simmer for 25–30 minutes until lentils are tender.
Add kale or spinach; cook for an additional 5 minutes until wilted. Stir in lemon juice and season with salt and pepper.