Season fish with some salt. Heat 2 Tbsp. oil in a medium Dutch oven over medium. Cook onion and garlic, stirring occasionally, until softened, about 5 minutes. Add cinnamon stick, garam masala, cumin seeds, and ½ tsp. pepper; cook, stirring occasionally, until fragrant, about 3 minutes. Stir in tomato purée, sugar, remaining 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt, and ½ cup water. Increase heat to medium-high; bring to a simmer. Reduce heat to medium, carefully add fish, and cook until fish is flaky but still firm, 8–12 minutes. Remove from heat; season with more pepper.