Soak the rice in cold water for 30 minutes, then rinse until the water runs clear. Put the rice into a saucepan and cover with 800ml water. Season and bring to the boil. Reduce the heat to a simmer, cover with a lid and cook for 10-12 minutes. When you remove the lid to check if the rice is cooked, all the water should have been absorbed. If not, cook for a couple of minutes more.
Put the saffron in a small bowl, add 1 tbsp water and leave for 5 minutes.
Stir the lemon juice and saffron water through the rice. Season.
Heat the oil in a saucepan over a medium heat. Add the mustard seeds and curry leaves and fry for 30 seconds. Add the mushrooms and onion, season and cook for 4-6 minutes.
Pour the mushroom mixture over the rice, cover with the lid again and cook for 5 minutes over a low heat.
To serve, stir the mushrooms through the rice, transfer to a platter and scatter over the lemon zest.