Heat the oil and fry the onion, ginger, garlic and chilli for 5 mins until starting to soften. Add the spices, curry leaves and potato, then cook for 1 min more. Stir in the coconut milk and tomatoes, cover and leave to simmer for 10 mins until the potato is tender.
Add the spinach and prawns. Cook for 1 min more until the spinach wilts and the prawns turn pink.
Meanwhile, make the rice. Tip the cumin into a pan and toast over a dry heat for 30 secs. Add the rice, salt to taste and 400ml water, then cover and cook for 8-10 mins until the rice is tender and the water has been absorbed. Serve with the curry.