In a deep pan heat 1 tbsp of ghee or oil and fry one chopped onion over medium heat for 10 minutes. Then add the grated ginger and the curry paste, cook while stirring for a minute or until the aroma is released.
Add the chicken to the pan and cook until the outside of each piece is sealed but not cooked all the way through. Then add the canned tomatoes, water and stir to combine, then lower the heat and simmer for 20 minutes covered with a lid. Then add the yogurt and cook on medium heat uncovered for 10 minutes longer to cook down some of the liquid.
Cook the rice in a rice cooker according to the rice cooker instructions. I like adding 3-4 cardamom pods and 1/4 tsp cumin seeds to the water for extra fragrance. This is is a n optional extra. (Alternatively you can use pre-cooked rice packets if short on time.)
In a separate pan fry the remaining sliced onion in hot oil until browned and are becoming crispy. Watch carefully and remove from the oil using a slotted spoon just when the onions start to get golden. If you wait any longer, they will burn. Remove from the oil and spread on a paper towel lined plate. Set aside till later.
If using saffron, steep in warm milk for for 20 minutes. If using turmeric, simply mix it with 2 tbsp of water. Set aside till later.You can also infuse the saffron in warm water if you are avoiding dairy.
Preheat the oven to 150C/300F.
Coat an oven proof dish with ghee, butter or oil, then spread the bottom of the pan with 1/2 of the cooked rice, top with the chicken curry, then finish with the remaining half of the rice.
Poke a whole in the centre of the dish with a long chopstick or the handle of a wooden spoon and drizzle the saffron milk along the sides of the tunnel or you can simply drizzle it all over the top of the rice.
Scatter the fried onions over the top of rice with little blobs of butter all over, cover with a lid or tin foil and bake in the preheated oven for 50-60 minutes. Gently mix the layers of your biryani before serving. Serve with raisins and toasted sliced almonds.