Wash the fish and pat them dry with a kitchen towel.
Rub the fish pieces with all-purpose flour, Bengal gram flour, salt and 1/2 tsp turmeric powder.
Heat 1 tbsp of mustard oil and when the oil is smoking hot, slide the fish pieces one by one in the frying pan.
Don't overcrowd the pan. Keep the heat medium so that it doesn't burn. Flip gently when one side has become crispy.
Take the fish pieces out of the pan and on a paper towel. Once they have cooled down a bit, remove the centre bone and break the fish pieces roughly with your fingers into smaller pieces.
Now peel the potato and cut into think strips (like potato sticks).
In the same frying pan, heat more oil and add sliced onions, crushed garlic and fry on high heat. When the onions have softened, add ginger paste.
Keep frying and when the raw smell has gone, add potato sticks and the chopped tomatoes.
Add salt, turmeric powder, red chili powder and stir-fry on high heat for 2 to 3 minutes.
After that, add cumin and coriander powder and stir. Cover and let the potatoes cook in the spices. Lower the heat so that nothing sticks to the bottom of the pan.
Once the potatoes have cooked, increase the heat and make sure that all the water has evaporated (if there is any from the tomatoes). Once the oil starts to release from the sides, sprinkle garam masala powder and the fish pieces.
Gently stir fry the fish pieces with the spices and the potatoes.
Keep the heat high. Add chopped green chilies and chopped coriander leaves to finish.
Serve it with steamed rice or roti.