Heat oil in a deep pan and add mustard seeds, curry leaves, and garlic and let it start to sizzle. Add in sliced onions and let brown slightly.
Toss in spices, chopped tomatoes and salt, and stir-fry for a few minutes till tomatoes begin to soften and pulp.
Add in a cup of water, cover with a lid and let it come to a slight boil. Add in fish pieces and continue to cook for a few more minutes till fish is almost cooked through.
Stir in tamarind paste and cook covered for another 3-4 minutes till the fish has soaked in all the flavours. You can add in a bit water at this point depending on the consistency of the curry you'd prefer.
Garnish with chopped coriander and serve warm over steamed rice.