Fry half the onions for 5 minutes until softened. Add the garlic, turmeric, chilli and ginger and fry for 2-3 mins.
Add the red lentils, turnips, carrots, tomatoes and hot water and bring to the boil. Cover and simmer for 20 mins until the vegetables and lentils are tender.
Add the garam masala and coriander and stir well. Blend the soup until smooth.
Meanwhile, heat the butter in a pan, add the remaining onion and ground cumin and cook for 5 mins until the onions are coloured and soft. Stir this through the soup while it is being heated.