Season fish with some salt. Warmth 2 Tbsp. oil in a medium Dutch oven over medium. Cook dinner onion and garlic, stirring infrequently, till softened, about 5 minutes. Add cinnamon stick, garam masala, cumin seeds, and ½ tsp. pepper; put collectively dinner, stirring infrequently, till fragrant, about 3 minutes. Lope in tomato purée, sugar, final 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt, and ½ cup water. Enlarge heat to medium-excessive; bring to a simmer. Decrease heat to medium, in moderation add fish, and put collectively dinner till fish is flaky however peaceable firm, 8–12 minutes. Steal faraway from heat; season with more pepper.
