How to Make It
Heat oil in a large pot over medium-high. Add onion, ¼ teaspoon salt, 1 tablespoon parsley stems, and half the garlic; cook, stirring occasionally, until softened, about 5 minutes. Stir in coriander and paprika; cook, stirring, until fragrant, about 1 minute. Stir in reserved tomato liquid and simmer until mixture thickens slightly, about 4 minutes. Stir in tomatoes, ½ teaspoon salt, and 1 cup water. Simmer over medium heat, partially covered, stirring occasionally and breaking up tomatoes with a wooden spoon, until flavors meld, 10 to 12 minutes.
Season fish with ¼ teaspoon salt and several grinds of pepper. Nestle fish into tomato mixture. Cook, covered, until fish is just cooked through, about 5 minutes.
Meanwhile, for gremolata, mix lemon zest and remaining garlic, 1 tablespoon parsley stems, and ½ teaspoon salt in a small bowl.
Ladle stew into bowls; top with gremolata and drizzle with oil.