Make shallow cuts across the skin side of 500 g fish. Place into a shallow dish, rub all over with the marinade spice and salt to taste, then cover and chill in the fridge for 30 minutes.
Blend 150 g onion and 12 g ginger with a splash of water to form a smooth paste.
Heat two-thirds of the ghee (about 26 g) in a large, heavy skillet over medium heat. Fry the fish for 1 to 2 minutes on each side until lightly golden, then transfer with a slotted spatula onto kitchen paper.
Melt the remaining ghee (about 13.6 g) in the pan over high heat, then drop the flame. Add the onion and ginger paste along with the ground spices for the sauce and fry for roughly 2 minutes until the spices deepen in colour.
Stir in 1 tablespoon vinegar, 4.2 g granulated sugar, and salt to taste. Add 18–24 g green chilli and stir-fry for 2 minutes.
Pour 125 ml of water into the skillet. Lower the fried fish back into the sauce, spooning the liquid over the top, then cover and gently simmer over low heat for 5 minutes until fully cooked.
Steep a tiny pinch (0.1 g per 2 g turmeric) in 2 tbsp warm water; use to match the rich golden hue. Contributes an aristocratic, honeyed floral flavour rather than earthy bitterness.
Use 80% of the weight, finely shredded and browned slowly with a pinch of asafoetida. Replicates the bulk and natural sweetness in gravies without allium.
Use 120% of the weight; melt gently over low heat to avoid burning the milk solids, which will yield a softer, creamier richness rather than a nutty aroma.