I recommend using a good Goan Fish Curry paste. In any Indian Shop you can always find numerous varieties of these pastes. If you have the luxury of living near a number of Indian groceries, look for the very best Goan Fish Curry mix by the brand, Parampara.
1. Place saucepan on heat. When hot, add some Virgin Olive Oil. When that's hot, throw in chopped up garlic, ginger and onion. This step is important, as the paste never get that bottom end without frying these veggies first. cook on medium heat until the onion goes light brown.
2. Add in the Coconut Milk. The only Coconut Milk you should use is the premium sort where no extra water is added nor has been fat-reduced. In the supermarket you will find many varieties of Coconut Milk. Basically you are looking for that which has the Coconut Cream retained. If you use any reduced fat or water diluted variety with Indian cooking, the spices are too powerful, and the result is often unpleasant to the palate.
Some studies have been done on this, due to Coconut Cream's high saturated fat content, but usually they find that because cultures which consume more Coconut Cream also mix it with fish and vegetables, they don't show the negative impacts of saturated fat. Personally, I think it best to not use it too much, but don't devalue a good dish with watered down Coconut Milk.
When you open a can of Coconut Milk, you will see the Coconut Cream congealed to the top or sticking on the lid - that's the difference between the Milk and the Cream, just like with cow's milk.
You don't need a lot of Coconut Milk - about a third of a cup is enough for one dish.
3. Mix in the curry paste. Simmer. The right amount of paste-to-milk is only discerned by trial and error, so best to experiment before foisting it on others.
4. The veggies. Indians will not usually add veggies to this curry, because their cooking is typically for a large number of people. Thus they will have separate curries for each vegetable or a combination of two vegetables - rarely ever more than that, unless it subzi. But if you are cooking for one or two, then it's impracticable to make up more than one main dish.
I have two versions. If you want to 'extend the fish texture' add in mushrooms and/or eggplant. If you have plenty of fish, and are looking for contrast, leave these out.
Other vegetables that go well are greens and reds. Greens: beans are always a favourite, and broccoli. Reds: red capsicum added in last is excellent. Peas are not so good, and better used in a separate dish if you want.
5. Fish. I prefer Nile Perch. Often a farmed fish that is environmentally unfriendly, but it has the required characteristics of a fish-for-curries. Namely, it holds together well in cooking, and is not too oily. Also, any delicately or expensively flavoured fish gets lost in the curry spices, so you have to remember it's the final dish that matters, not the fish itself.
Any fish which satisfies these characteristics is good.
6. Add these in, in sequence - remember the fish falls apart with too much cooking, so I typically add it in last, with only the red capsicum to go in just before serving, to keep that crisp.
A bit of pickle is always good on the side, and you can't do much better than a nice lime pickle with fish.