Marinate the fish pieces with harissa and salt. Refrigerate for at least one hour. Shallow fry in vegetable oil and keep aside.
Place baking paper in a tray and add the potato slices. Season with salt and oven roast till ¾ ths done (200°C for 20 minutes). Alternately, you can also shallow fry the potato slices, drain and keep aside.
To prepare the sauce, heat oil in a pan and add cardamom pods. Add the garlic, green chilli and chopped onions. Saute till onions are light brown and then add turmeric powder and ground cumin powder.
Add the tomatoes and sauté till the saucy consistency is achieved. Season with salt and add a little water if necessary but ensure that the sauce is thick and not runny.
Cook the rice in boiling water seasoned with salt, drain and keep aside. Make sure that the rice is just cooked so that the grains remain separate and not mushy.
In a large pot, add one layer of rice followed by a few fried fish pieces, potato slices and sauce/kushna. Continue the process by adding and building up the layers of rice, fish, potato and sauce. Finally garnish with chopped coriander leaves.