In a large pot, heat 3 tbsp oil over high heat until hot and shimmering. Add onion and saute until golden, about 4 minutes, and garlic and saute for 1 more minute.
3 tbsp olive oil, 1 onion, 2 cloves garlic
Stir in tomato paste, cumin, salt, black pepper and chili powder or cayenne, and saute for 2 minutes longer.
1 tbsp tomato paste, 1 tsp cumin, ¼ tsp salt, ¼ tsp pepper, ¼ tsp chili powder, ¼ tsp cayenne powder
Add broth, 1½ cups water, lentils and carrot. Bring to a simmer, then partially cover pot and turn heat to medium-low. Simmer until lentils are soft, about 30 minutes.
4 cups chicken broth, 1 cup lentils, 1 carrot
Taste and add salt if necessary.
Using an immersion or regular blender or a food processor, puree half the soup then add it back to pot (soup should be somewhat chunky).
Stir in lemon juice when ready to serve.
1/2 lemon