Peel sweet potato and cut into small cubes (about 1/2"/1cm).
Peel onion, cut in half and add a toothpick to the onion to keep it from falling apart in the soup. The onion will be removed as one at the end.
Peel and crush garlic.
Add lentils, sweet potato cubes, half onion and crushed garlic, cumin and about 1 tsp salt to boiling water, cover and reduce heat to simmer.
Simmer for approximately 30 minutes or until sweet potato is very soft.
In the meantime, prepare coriander oil by picking a big hand full or coriander leaves and adding to blender together with olive oil. Blend well until all coriander is very small. Scraping might be necessary mid-through.
Once soup is cooked, remove half onion, blend remaining ingredients with an immersion blender or blender or food processor and season to taste with more salt and pepper.
Serve in a bowl with a swirl of coriander oil and freshly ground pepper and fresh naan bread or bread of your choice.