Cut the fish fillets into 1 inch pieces and keep in a large bowl.
Chop the onions roughly and set aside
Squeeze the juice from the lemon onto the fillets and let it sit while you get the rest of the ingredients together (Tip: The lemon juice helps ensure the fish isn't "smelly" when you cook it. You can also use milk instead of lemon juice).
Chop cilantro leaves and keep aside.
Bring a large pot to medium-high heat. Add mustard oil and vegetable oil and wait for it to heat up.
Add the mustard seeds to the oil and let them splutter.
When the seeds have stopped spluttering, add the chopped onions and sauté for ~12 mins until the onions are cooked and you can’t smell the rawness of the onions anymore.
Add garlic paste and ginger paste and sauté for two minutes.
Then add tomato paste, red chili powder, coriander powder and fish curry masala powder and stir well to let the spices combine. Let it cook together for a few minutes.
Switch stove off and let the mixture cool down for five minutes.
Use an immersion blender and blend the mixture nicely till it is a smooth paste. You can add up to 1 cup of water to help with the blending.
Now add the tamarind paste along with 1.5 cups of water and bring the stew to a boil.This is one of those recipes where you will need to add salt to taste because it depends on how strong your tamarind paste is. You probably won’t need more than 0.5 tsp salt.
Add the fish to the stew and close the pot. It will take 6-8 mins for tilapia to cook. Be careful about not overcooking the fish.
Once the fish is cooked, bring the pot to low heat. Add yoghurt and mix well. Your stew is ready! Garnish with cilantro leaves and serve.