Drain, add water and cook with the turmeric, bay leaf, and salt, skimming occasionally, for about 1 hour, until soft.
Purée the whole pot, using an immersible blender, or remove some lentils and mash to adjust the consistency and thickness preferred for the final soup.
Adjust the seasoning to taste.
To Make the Spiced Oil
Fry the cumin seeds in the oil then add the ginger and garlic, cooking until browned.
Add the chili powder then a little water to stop the oil cooking further.
To Serve
Season the soup with a drizzle of the flavoured oil, coriander and add a sprinkling of the lemon juice.
Notes
Extra spiced oil can be stored in the fridge and used as a boost to final seasoning for other dishes, such as vegetable stir-fries.