In a bowl, add the chilli powder, turmeric powder, ginger and garlic paste, cumin seeds powder and salt, mix thoroughly.
Divide the paste into two halves, one portion is used for marinating fish, the other half is used for the dry masala.
Marinade the fish pieces with one half of the paste along with lemon juice and carom seeds/ajwain seeds powder.
Keep marinated for 10 to 15 minutes.
Heat 2.5 tbsp of oil in a non-stick skillet, shallow fry the fish pieces till it turns a medium golden brown colour on both the sides, keep aside.
In the same skillet, heat 2 tbsp of oil, add the chopped onions and fry till translucent, add the rest of the masala paste along with fish, garam masala powder and salt, if required, fry for 3 to 4 minutes or until the raw smell of the spices vanish.
Add 2 tbsp of the tamarind pulp and cook for a minute or two by constant stirring.
Lastly, slide in the fish pieces, and let cook covered for another 5 minutes on medium heat until all the flavours blend well.
Remove skillet off the heat.
To temper, heat a little oil, add the curry leaves and chopped green chillies.
Add the tempering to the fish masala, serve hot with rice/chapattis.