In a large pot over high pot, heat the oil. Add onion and garlic, and sauté until golden.
Stir in tomato paste, cumin, salt, black pepper and chili powder or cayenne, and sauté for 2 minutes.
Add broth, lentils and carrot. Bring to a boil, then partially cover pot and turn to medium-low. Simmer until lentils are soft, 30 or more minutes depending on altitude.
Use an immersion blender, half-purée the soup. Leave it a little chunky
Stir in lemon juice.
Serve with cilantro (if you wish) and drizzled with olive oil. Can also dust lightly with chili powder if you wish.