Soak mustard seeds, garlic and poppy seeds in warm water for 10 minutes. Grind poppy seeds, mustard seeds and garlic cloves into a fine paste and keep it aside.
Wash, peel and cut the potatoes into big pieces. Keep them aside in a water-filled bowl to prevent them from turning black.
Clean and wash fish properly. Marinate with ½ teaspoon of turmeric, red chilli powder and salt. Add potato pieces to the marinated fish and mix it well. Keep it aside for 5 minutes.
Now heat mustard oil till smoking in a heavy-bottomed pan or kadhai and deep fry all the fish pieces carefully from both sides on medium heat till golden brown.
Deep fry the potato pieces until crisp and golden brown and set them aside.
Next, heat 3 tablespoons of mustard oil till the smoking point. Now keep the flame low and add hing, panch phoran to the oil.
Add turmeric powder and degi mirch powder to the oil. Add finely chopped tomatoes, salt and slit green chillies. Cook the tomatoes for 4-5 minutes till they turn soft.
Next, add mustard seeds paste, poppy seeds paste and cumin powder. Cook this spice mixture until the oil starts to separate from it.
Add 2 cups of water and bring it to a boil. Turn the flame low and add the fried fish pieces and potato pieces. Let it simmer for 4-5 minutes.
Switch off the flame and add coriander leaves.
Serve it with hot steamed rice.