Saute the onion for about 5 minutes over a medium heat until translucent and just starting to brown.
Add the garlic and ginger, and cook, stirring gently for 1 minute.
Add the curry powder, turmeric and curry leaves, and cook, stirring for a further minute until the mixture is fragrant.
Slowly stir in the coconut milk, scraping up the spices and onion from the bottom of the saucepan. Bring the pan to a gentle simmer.
Add the chopped tomato, and continue to simmer for about 5 minutes or until the tomato begins to soften.
Add the fish and the salt to the sauce, and gently poach for about 6 - 8 minutes, or until the fish is cooked.