1ltr/1¾pt cold water
1 large white onion, peeled and roughly chopped
2.5cm/1in piece root ginger, peeled and finely chopped
1 tsp turmeric
1 tsp cumin seeds, roughly crushed
200g/7oz canned chopped tomatoes
300g/11oz red lentils
Salt and freshly ground black pepper
For the topping
2 tbsp vegetable oil
2 white onions, peeled and thinly sliced
1 clove garlic, peeled and finely chopped (optional)
1 tsp cumin seeds, roughly chopped
2 tsp black onion seeds
¼-½ tsp dried crushed red chilli
Pinch of turmeric
150g/5oz natural yoghurt
Small bunch of fresh coriander leaves, torn, to garnish
Pour the water into a mediumsized pan. Add the chopped onion, ginger, turmeric, cumin seeds and chopped tomatoes, then bring to the boil.
Place the lentils in a sieve, rinse with cold water, drain, then add to the saucepan with plenty of salt and freshly ground black pepper. Bring back to the boil, then simmer for about 30 minutes, stirring from time to time, until the lentils are soft.
About 10 minutes before you are ready to serve, make the topping. Heat the vegetable oil in a frying pan. Add the sliced onions and fry gently for about 5 minutes, until softened. Add the garlic, if using, the cumin and onion seeds, the crushed chilli and turmeric, and fry for 3-4 minutes more, stirring until browned.
Spoon the lentils into individual serving bowls, top with a spoonful of yoghurt and a spoonful of the onion mixture, then garnish with torn coriander leaves. Serve with warmed naan bread or chapattis, if liked.