If using goraka, rinse it well and soak in warm water for about 15 minutes. If using tamarind paste, skip this step.
In a mortar and pestle or a spice grinder, grind the black peppercorns into a coarse powder.
Combine the ground black pepper, ground turmeric, and soaked goraka (or tamarind paste) to form a thick paste. Add a little water if needed.
Rub the spice paste all over the fish chunks, ensuring they are well coated. Let it marinate for at least 30 minutes.
In a large pot, heat the coconut oil over medium heat.
Add the finely chopped onion, minced garlic, grated ginger, curry leaves, and slit green chilies. Sauté until the onions are translucent and fragrant.
Add the fenugreek seeds, mustard seeds, and cumin seeds to the pot. Sauté for another 1-2 minutes until the seeds start to pop.
If using chili powder, add it now and mix well.
Gently add the marinated fish chunks to the pot, placing them in a single layer if possible.
Pour in the water and bring the mixture to a boil.
Reduce the heat to low, cover, and let it simmer for about 20-30 minutes, or until the fish is cooked through and the sauce has thickened. Be careful not to stir too much to avoid breaking the fish pieces.
Taste and adjust the seasoning with salt if needed.
Remove from heat and let it rest for a few minutes before serving.
Enjoy your meal!