Season the fish pieces on all sides using turmeric, red chili powder, and salt.
Then let the fish stand for 15-minutes.
In a deep-sided skillet, heat oil for frying until a bit of flour dropped into the oil bubbles.
Place the fish into the oil making sure not to overcrowd the pan. Cook for 3 minutes per side or until cooked through and golden browned.
Remove from the pan and set aside on paper to drain.
In a fresh skillet, add mustard oil, mustard seeds, and fenugreek seeds. Cook for 1 minute until they begin to crackle and are aromatic.
Next, add the green chiles and saute for another 1 minute.
Add in the turmeric and red chili powder, then saute for 1 minute.
Add the cooked potatoes to the skillet and saute, stirring regularly, for 4 to 5 minutes.
Add water to the mixture and stir to combine so the gravy forms.
Once it begins boiling, add the fried fish and let simmer for 5 minutes.
Add more or less water until it is your desired consistency.
Salt to taste and then add lemon juice and stir.
Garnish with freshly chopped coriander. Serve alongside steamed rice with your favorite garnishes.