In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté for about 5 minutes, until translucent.
Stir in the minced garlic, carrots, celery, and bell pepper. Cook for an additional 5-7 minutes, until the vegetables start to soften.
Add the dried thyme, oregano, cumin, and smoked paprika to the pot. Stir well to combine and cook for 1 minute until fragrant.
Pour in the diced tomatoes with their juice, rinsed lentils, and vegetable broth. Bring the mixture to a boil.
Reduce the heat to low, cover, and let it simmer for about 30-35 minutes, or until the lentils are tender.
Stir in the chopped kale or spinach and cook for an additional 5 minutes until the greens are wilted.
Season with salt and pepper to taste. If desired, stir in the lemon juice before serving.