Heat olive oil in a large pot over medium heat. Add carrots, celery, and onion and cook until softened, about 5-7 minutes.
Add lentils, vegetable broth, thyme, and smoked paprika to the pot. Bring to a boil, then reduce heat and simmer for 20-25 minutes, or until lentils are tender.
Season with salt and pepper to taste. Stir in lemon juice, if desired.