Wash the lentils well and place in a heavy saucepan. Cover with the water. Meanwhile, if using frozen spinach, remove from the freezer and allow the block to begin to thaw. If using fresh spinach, wash well and chop finely. Peel the onion and garlic and coarsely slice them. Add to the lentils. Cover and bring to the boil, then add the ginger, chili powder and the salt and simmer gently.
Cover the saucepan and allow to simmer until the lentils break down and turn into soup. Once this has been achieved, boil off any excess water so that quite a thick sauce is made. Add the spinach. If this is not totally thawed out by now do not worry, continue to cook until the spinach is thawed and totally mixed in with the lentils. Finally, add the yogurt and stir well in. Cook for a further 3 minutes and serve.