Put the red split lentils in a bowl and rinse them in several changes of cold water until the water is much less cloudy. Drain thoroughly.

Put the drained lentils, 650ml water, onion, tomato, garlic, ginger, green chilli, 10 curry leaves, turmeric, ground cumin, ground coriander and hot chilli powder in a medium saucepan. Bring to the boil and skim off any foam. Reduce the heat and simmer partially covered for 15 to 18 minutes, stirring every few minutes, until the lentils are tender and beginning to break down.

Stir in the coconut milk and 1¼ teaspoons salt. Simmer gently, uncovered, for 5 to 7 minutes, stirring regularly. Add up to 100ml hot water as needed to maintain a creamy, spoonable consistency.

Heat the oil in a small frying pan over a medium heat. Add the red onion and cook for 4 to 5 minutes until softened and lightly golden. Add the mustard seeds and cumin seeds. When the mustard seeds begin to pop, add the dried chilli and 8 curry leaves. Cook for 20 to 30 seconds, then immediately remove the pan from the heat.

Stir about three quarters of the tempering into the lentils. Taste and add more salt if needed. Transfer the dal to a serving bowl, spoon over the reserved tempering and serve hot.