Drain the goraka, reserving the soaking liquid, and grind it with black pepper, turmeric, coriander, salt, and enough soaking liquid to make a thick paste.
Coat the tuna gently with the paste and marinate for 30 minutes.
Arrange the Pan:
Scatter onion, garlic, ginger, curry leaves, and cinnamon across the base of a wide pan. Arrange the fish in one layer over the aromatics.
Add water around the fish and bring to a gentle simmer over medium heat.
Cook Nearly Dry:
Cover and cook over low heat for 15 minutes. Uncover and simmer 15 to 20 minutes more, turning the pieces once very carefully, until the sauce is thick and nearly dry.
Cool for 10 minutes before serving so the coating sets on the fish.
Notes
Goraka provides the characteristic dark sourness; tamarind changes the flavor and should be a last-resort substitute.
Choose firm, thick tuna pieces so they hold their shape during reduction.