Meanwhile, wash the chana dal in cold water. In a small saucepan, bring 225 ml / 8 fl oz of salted water to the boil, add the chana dal and cook, covered, for 10-15 minutes until well cooked. Set aside.
Using a spice mill or pestle and mortar, grind the coconut, green chilli and fennel to a fine paste.
In a large frying pan, heat the oil. Add the mustard seeds and as they begin to pop, add the onion and curry leaves. Stir well and cook over a medium heat until the onion is golden brown. Add the ground coconut mixture, then reduce the heat and cook, stirring for 2-3 minutes. Mix in the cooked papaya and chana dal and serve hot.