1. Heat the olive oil in a large pot over medium heat. Add the onion and garlic, and sauté until softened.
2. Add the cumin and coriander, and cook for another minute.
3. Add the sweet potato, lentils, and vegetable broth. Bring to a boil, then reduce heat and simmer for 20-25 minutes, until the sweet potatoes and lentils are tender.
4. Use a blender to puree the soup until smooth. Season with salt and pepper to taste.