Clean and marinate prawns: Peel and devein 500g large prawns. Mix with 1 cup yogurt, 1.5 tbsp ginger-garlic paste, 2 tsp red chilli powder, half tsp turmeric, 1 tsp biryani masala and salt. Marinate 1 hour.
Fry birista: Slice 3 large onions thinly. Deep fry until very dark brown and crispy. Drain. Reserve oil.
Parboil basmati rice: Wash 2 cups basmati. Soak 20 minutes. Boil in salted water for 7 minutes. Drain at 70% cooked.
Cook prawn masala: In a heavy pot heat reserved birista oil. Add 3 chopped tomatoes and cook 8 minutes. Add marinated prawns and cook 5-6 minutes until just pink.
Layer biryani: Spread prawn masala as base. Sprinkle half the birista.