Heat the olive oil in a large pot or Dutch oven over medium heat. Add the chopped onion, carrots, and celery and cook until softened, about 5-7 minutes. Stir in the minced garlic, dried thyme, smoked paprika, and ground cumin, and cook for another minute until fragrant.

Add the rinsed lentils, diced tomatoes (with their juices), and vegetable broth to the pot. Add the bay leaf and season with salt and pepper. Bring to a boil, then reduce the heat to low, cover, and simmer for 30-35 minutes, or until the lentils are tender.

Remove and discard the bay leaf. Stir in the fresh spinach until it wilts, about 2-3 minutes. Turn off the heat and stir in the fresh lemon juice. Taste and adjust seasoning with more salt and pepper if needed. Serve hot.