Heat enough oil in a wok to deep-fry salted fish slices until golden brown. Drain well and set aside.
Heat 4 tbsp oil and stir-fry sliced shallots and garlic until limp. Add mustard seeds, fenugreek and curry leaves and saute until fragrant. Add the rest of the spice ingredients and stir-fry well.
Blend in vinegar, tamarind juice and sugar. Bring to a boil then simmer until gravy is thick and the oil has separated.
Cool the gravy then stir in salted fish pieces. Add salt to taste. Sprinkle with 1 tbsp toasted sesame seeds. Dish out and keep in clean, dry bottles.