I did a bit of hunting...both in my cookery books,and on the web. A 'masala' is simply a mixture of spices.....either dry,or as a paste. If a paste-I would rub the fish with it,then leave to marinate for several hours....this if you are cooking a whole fillet. But if using chunks of fish....such as cubed cod loin or monkfish...I would cook the paste first...then-when the spices are amalgamated and less 'raw' I would add the fish. This may be the better option as fish does not have a long cooking time.
Madhur Jaffrey has a recipe in one of her books for Masala Steak.....this has a paste made up of ginger,garlic and onion. There is also a paste,which includes ground coriander,cumin seeds,mustard seeds,turmeric,and cayenne pepper.