Drain the tomatoes in a sieve over a heatproof measuring jug, then set aside. Bring a kettle of water to the boil.
Heat the oil in a large flameproof casserole dish over a medium heat. Add the onion, chilli and ginger and stir for 3 minutes. Stir in the cumin, coriander, turmeric and nutmeg and continue stirring until the onion is softened.
Add the rice and drained tomatoes to the casserole dish and stir to mix with the spices. Add enough boiling water to the reserved tomato juice to make up to 500ml. Stir this liquid into the rice with the sugar and a pinch of salt, then bring to the boil.
Reduce the heat to low, cover tightly and leave the rice to cook without lifting the lid for 10–12 minutes until all the liquid has been absorbed, the rice grains are tender and tiny holes appear on the surface.
Stir in the spinach, as much as you can at a time, adding more as each addition wilts. When all the spinach has been added, lay the prawns on top, re-cover the casserole dish and turn down the heat to very low. Cook for 2 minutes, then turn off the heat and leave to stand for 1 minute, without lifting the lid, by which time the spinach will have wilted further and the prawns will have cooked through. Gently fork together to combine the rice, spinach and prawns. Serve immediately.