Heat the oil in a large saucepan.
Add the mustard seeds. When they sputter and crackle, add the curry leaves and green peppers. Add the turmeric and mix well for 20 seconds.
Add the rice and saute until the grains start to turn opaque, no more than a minute or two.
Add the mango puree, cashew nuts, biryani masala, and water, stir well, and bring the mixture to a boil. Lower the heat the a simmer, and put on a tight lid. Cook for 20 minutes, undisturbed. Let the rice stand at least 10 more minutes after cooking, without taking the lid off, to ensure it is fully cooked.
With a fork, fluff the grains of rice and serve hot with a spicy dal or sabzi.