Prepare mushrooms: Clean 400g button mushrooms by wiping. Slice thickly. Marinate with half cup yogurt, 1 tsp red chilli powder, half tsp turmeric, 1 tsp biryani masala and salt for 30 minutes.
Fry onions: Slice 2 large onions. Fry in 3 tbsp oil until golden brown. Drain on paper.
Parboil basmati rice: Wash 2 cups basmati. Soak 20 minutes. Boil in salted water for 7 minutes. Drain at 70% cooked.
Cook mushroom base: In a heavy pot heat 2 tbsp oil. Add 2 chopped tomatoes and cook 6 minutes. Add marinated mushrooms — they cook quickly, about 5-6 minutes.
Layer biryani: Spread cooked mushrooms as base. Add parboiled rice on top.
Add garnishes: Add fried onions, 3 tbsp ghee, fresh mint and coriander.
Add saffron milk: Drizzle saffron milk over the rice.
Seal and dum cook: Cover tightly and cook on very low heat 20-25 minutes — mushrooms cook faster so less time needed.
Rest: Rest covered 8 minutes.
Serve: Mushroom biryani has a rich earthy flavour. Serve with raita and sliced onion.