Boil fish in water for 8-10 minutes. Cool and drain. Remove skin and bones and mash fish flesh. Add chopped onion, ginger, chilli, coriander leaves, egg, garam masala, salt, black pepper and lemon juice. Beat or grind into a smooth paste. Make 10-12 equal-sized portions and pat into a flat burger shape. Heat oil and shallow fry on each side for 3-4 minutes. Serve with onion salad.