Heat oil in a large saucepan over medium heat. Add onion, carrot, and coriander roots and stems. Cook, stirring occasionally, for 8 minutes or until vegetables soften. Add garam masala and sweet potato. Cook, stirring, for 2 minutes or until fragrant. Add stock and 2 cups water. Cover and bring to the boil. Reduce heat to low and simmer, partially covered, for 25 minutes or until sweet potato is tender when pierced with a fork. Remove from heat.
Using a stick blender, blend soup until smooth. Season with pepper. Stir in three-quarters of the lentils. Ladle soup among bowls. Top with a dollop of yoghurt, remaining lentils and coriander leaves. Season with pepper and serve.
First time I made this soup I used 2 cups stock but omitted the 2 cups water. It was thick, hearty and delicious. The second time I used 2 cups stock + the 2 cups water....it was very thin with very little body. Both times I used one cup dried lentils, cooked & drained. I didn't have fresh coriander so added a handful of dried coriander leaves with the liquid, To thicken the watery version I stirred in a heaped tbls of LSA + a heaped tbls of savoury yeast flakes, This gave it the soup the constancy of my first version but diluted the spicy taste so I stirred in a few drops of hot pepper sauce!