Heat 150ml of vegetable oil in a large frying pan over a moderately high heat. When almost smoking, add the onions and a pinch of salt and cook for 20-25 minutes, until golden and crispy. Remove with a slotted spoon onto kitchen paper to drain. Retain the oil.
Pour half the retained oil into a large saucepan and set over a medium heat. When hot, add all the ingredients from the spice base. Season with a little salt and cook for 5 minutes until fragrant. Add 300ml of warm water and the split peas, stirring well. Cover with a lid and simmer gently for 25 minutes, stirring regularly. Add the coconut milk and simmer for a further 5 minutes.
While the spice base is cooking, bring a large saucepan of salted water to the boil. Add the rice and part-cook for 6 minutes. Drain well.
Put the saffron in a small bowl and pour over 50ml of boiling water.
Line the base of a medium to large saucepan with a circle of parchment paper. Put 25ml vegetable oil, plus the butter, on top of the paper and set over a moderately high heat. When hot, layer a third of the rice base. Spoon over a little of the saffron water. Layer half the split pea mix on top, followed by half the cavolo nero leaves and a third of the crispy onions. Repeat the layers. Top with the remaining third of rice, the saffon water and onions. Cover with a firmly fitting lid and cook over a low-medium heat for 40 minutes. Don’t be tempted to lift the lid and check while cooking, as the steam will escape, which is needed to cook the rice.
Check the biryani after 40 minutes to see if the rice is al dente. If so, it is ready. If not, leave for a further 10 minutes.
When ready, serve with coconut yoghurt and coriander alongside.