Take fish pieces in a plate. Add garlic paste, little bit salt and vinegar and spread it over the fish pieces covering all sides. Keep it aside for 10 to 15 minutes.
Prepare Gram (besan) powder mixture by adding in it salt, black pepper, crushed cumin seeds, dried fenugreek leaves and fish masala. Roll over the fish pieces nicely in the gram powder mixture covering all the sides.
Heat oil 2 - 3 tbsp and fry the fish pieces on low to medium flame. Don't forget to turn sides while frying the fish pieces.
Cut a medium sized onion in a separate plate and add crushed garlic, salt, turmeric powder, red chilies and tamarind pulp or juice whichever is available. Keep it aside for 5 - 7 minutes. Press the onion with hands to remove excess water and spread evenly on the fish pieces.
Cover the pan and cook on low flame for 8-9 minutes. When ready, sprinkle chaat masala, decorate with some green chilies and serve.