Wash the fish pieces 3 to 4 times and drain excess water. Add turmeric powder and keep aside for 20 to 30 minutes. Mix together the ginger, garlic and green chilli pastes. Heat 4 tblsp oil in a pan. Add the fish and deep fry for a minute or two. Remove and keep aside. In a separate pan, heat 1 cup of oil and fry the mustard seeds till it starts to splutter. Add the ginger-garlic-green chilli paste and fry till the oil floats to the top. Add the curry leaves, red chilli powder and stir-fry for 30 to 60 seconds. Add the fried fish, vinegar and salt. Add the mustard seeds, cumin powder and fenugreek powder. Bring to a boil and remove from heat. Transfer the pickle to a clean, sterilized jar and cover with a lid. Keep aside for 30 to 40 days before using. Do not refrigerate once opened.