Wash the rice very well in several changes of cold water, until the water runs clear. Place in a saucepan, then cover with warm water. Add salt, to taste, and bring to the boil and simmer for 10-12 minutes.
Put the lentils into a separate saucepan and cover generously with water. Bring to the boil, reduce heat to a gentle simmer and cook for about 10 minutes or until the lentils are still a bit hard. Drain immediately and set aside.
While the lentils are cooking, make the caramelised onions. Heat the oil in a large frying pan. Depending on the size of your frying pan, you may need to cook them in two batches. When the oil is hot, add enough sliced onion to make a single layer, and cook over a medium heat, stirring occasionally. Towards the end of the cooking, when onions are beginning to brown, stir constantly to stop them from burning. The cooking time will be about 30 minutes. When nicely browned, remove onions with a slotted spoon, and then fry the rest of the onions.
Melt butter in a saucepan and add the spices. Stir in the drained lentils and rice and a third of the fried onions. Gently toss them all together. Pour enough water into the saucepan to cover the rice and lentils by about 0.5cm/¼inch and season to taste with salt and freshly ground black pepper. Cover the top with some damp baking parchment cut to the shape of the pan or some foil so it touches the surface.
Put a lid on the pan and bring to the boil over a high heat. After five minutes turn down the heat to low, then cook for a further five minutes. All the water should now have been absorbed and the rice grains will be standing on end. Serve covered with the remaining fried onions and sprinkled with the chopped coriander.
