Heat the olive oil in a large pot over medium heat.
Add the chopped onion and garlic, and sauté until softened, about 5 minutes.
Add the carrots and celery, and cook for another 5 minutes.
Stir in the cumin, paprika, salt, and black pepper.
Add the lentils, diced tomatoes, and vegetable broth. Bring to a boil.
Reduce heat to low, cover, and simmer for 30-35 minutes, or until the lentils are tender.
Adjust seasoning if necessary. Serve hot.