Heat the oven to 240C (220C fan)/475F/gas 9.
In a large baking dish, combine the finely sliced onion and cauliflower, then add the oil, spices and salt, and rub them in to coat everything well.
Cover the dish tightly with foil, pressing all around the edges, then put to one side.
Put the rice, oil, harissa, ginger, salt and 700ml just-boiled water in a second large baking dish.
Cover this with foil, too, sealing it well and pressing down all around the edges.
Bake both trays for 25 minutes, carefully turning them around once halfway through.
Take both trays out of the oven and put the rice tray to one side, with its foil lid intact, and leave to rest.
Take the foil off the cauliflower tray and return it to the oven for 10 minutes to crisp up.
After 10 minutes, take the foil off the rice, then tip the cauliflower and onion mixture all over the top.
Pour over the lemon juice, sprinkle with the chopped herbs and cashews, and serve warm.
