Add the chopped tomatoes, followed by the yoghurt. This step is optional and depends on the amount of gravy in the leftover curries you are using.
Once the tomatoes are soft, add the biryani masala or use your own spices (see notes for details). Fry for a minute, and then add the potatoes (if using).
Add the leftover curry / salan, and cook for about 3 – 4 minutes till the meat and vegetables heat up and oil can be seen separating from the side. The potatoes should also be 80% done at this stage.
Once the leftover biryani masala mixture is ready, set it aside.
Layer ½ the biryani masala at the bottom, and then add ½ the rice. Top with the remaining biryani masala, followed by the rice.
On top of the rice, add the coriander and reserved fried onions, along with green chilies.
Place the pot on an insulator – this can be a tawa, a flat gridle or a heat diffuser (available at Asian and Indian stores). If you have a cast iron Dutch oven or another heavy bottomed pan, this diffuser can be skipped. Turn the heat to medium for about 3 – 4 minutes. This will help in creating the steam. Once the steam is formed, turn heat to low / simmer, and cook for about 15 minutes on ‘dum’ – this means that the rice will cook in its own steam. Turn off the stove, and let the biryani rest for 10 minutes.