Scrub and wash mushrooms, drain and cut into quarters. Pick and wash basmati rice in plenty of water and soak in sufficient water for half an hour. Peel and finely slice onions. Peel ginger, garlic and grind together to a fine paste. Clean, wash and finely chop fresh coriander and mint leaves.
Wash tomatoes and make a puree in a blender. Whisk the skimmed milk yoghurt and keep aside. Soak the saffron in one fourth cup warm skimmed milk. Boil three to four cups water in a thick bottomed vessel, add bay leaf, cloves, green cardamoms, black cardamoms, cinnamon, mace and one teaspoon salt.
When the water starts boiling rapidly, drain the soaked basmati rice and add. Cook for eight to ten minutes, stirring frequently or until the rice is three fourth done.
Drain in a colander. Heat oil in a non-stick pan, add sliced onion & stir-fry over high heat for two to three minutes. Add ginger garlic paste & cook briefly. Add red chilli powder, coriander powder, crushed peppercorns, cumin powder & turmeric powder.
Stir-fry briefly and add the pureed tomatoes. Continue cooking over high heat for another 2 to 3 minutes, stirring continuously or until the masala is fairly thick. Add the whisked skimmed milk yoghurt, garam masala powder & half the quantity of chopped fresh coriander & mint leaves. Stir well & cook for 2 minutes more.
Add the quartered button mushrooms & salt to taste. Stir-fry over high heat for two to three minutes & remove from heat.
Arrange cooked rice & mushrooms masala in alternate layers in an ovenproof dish, sprinkling chopped fresh coriander & mint leaves, kewra water & skimmed milk with saffron. Ensure that the topmost layer is of rice.
Cover assembled biryani with a tight fitting lid & seals the edges with kneaded atta dough. Keep the sealed dish on a medium hot tawa & leave for 10 to 15 minutes. You can also place a few burning charcoals on the lid.
Alternately, leave the sealed dish in a preheated oven for 10 to 15 minutes. Break the seal & open the biryani just before serving.