Warm a small skillet/frying pan over a medium heat and toast the whole spices (coriander, cumin and mustard seeds) for a couple minutes until fragrant. The mustard seeds may start to pop which is a sign to stop.
Roughly chop the garlic and ginger and crush with the salt to a relatively smooth paste. Add the toasted spices and crush. Add the remaining powdered spices and mix to a relatively smooth paste.
Finely dice the chilis and onion. Dice the tomato. Warm the oil in a medium-large skillet or other shallow pan over a medium heat.
Cook the onion in the oil for a few minutes until it has softened. Add the spice paste and stir through. Cook for a minute or two then add the tomato and chili and let the liquid almost disappear (a minute).
Cut the fish into large bite-sized chunks.
Add the tamarind and coconut milk to the pan, mix through. then put the fish chunks in and cook until cooked through, a couple mins.