Preheat oven to 180 degrees Celsius or 360 degree Fahrenheit.
Grease a thick bottom oven safe pan/ disposable aluminum tray with butter (ghee) or cooking oil.
In a separate pan, heat ghee (or butter). Add sliced onions and fry until they turn golden brown or caramelized. Remove from pan and drain into a bounty to remove excess ghee.
Heat milk and saffron in a small bowl. For convenience, use a microwave. Else, heat milk in a small saucepan and add saffron. Bring it to a boil and remove from heat. Keep aside.
Layer half of the cooked chicken with its cooked gravy and sprinkle half of chopped mint, cilantro and nuts. Next layer half of the 75% cooked rice.
Sprinkle half of fried onions, green chili, saffron milk, almonds and cashew nuts.
Layer the remaining cooked chicken along with it's cooked gravy.
Layer the remaining rice and sprinkle the remaining fried onions, green chili, cilantro, mint, saffron milk, almonds and cashew nuts. Also add the left over marination gravy.
You can make 2 layers of chicken and rice or 4 layers. Or more than that (depends on the depth of the container and the quantity you are cooking). But the top most layer should always be that of rice.
Seal the container with an aluminum foil. Bake the biryani for 20-25 minutes.
Serve hot and delicately flavored biryani with 'Dahi Ki Chutney' and 'Mirchi Ka Salan'. Enjoy!