Preheat the oven to 140 degrees C. Rub the fish with the salt, lemon juice and turmeric and let marinate 20 minutes. Sprinkle with chilli powder and 2 tablespoons chickpea flour and mix with your hands.
Heat the oil in a deep saucepan and deepfry the fish 3 minutes or until golden. Drain on paper towels and keep warm in the oven.
Make the sauce: heat the ghee in a saucepan, add onion and saute until golden. Add cumin, turmeric, chilli powder and salt and saute until you can smell the spices. Add ginger and green chillies and cook 2 more minutes.
In a bowl, whisk together the yoghurt and chickpea flour until smooth. Add yoghurt to the spices and bring to a simmer. Add the water and simmer 5 minutes.
Add the fish to the sauce and simmer 5 more minutes. Stir in the garam masala. Serve!