In a large saucepan, heat the oil. Add the onion and garlic and cook over moderately high heat until fragrant, about 2 minutes. Add the celery and carrot and cook over moderate heat for 5 minutes. Add the potato, lentils and stock; bring to a boil. Cover and simmer until the vegetables are very tender, 40 minutes.
Puree the soup in batches and return it to the saucepan. Add the cumin, cayenne and lemon juice and season with salt and pepper. Ladle the soup into bowls and serve.