Preheat the oven to 350 degrees . Place the fish in a single layer in a large glass baking dish; season with salt and pepper and 2 tablespoons olive oil. Bake for 8 minutes; set aside.
Increase the oven temperature to 425 degrees . Place the bread cubes on a baking sheet, drizzle with olive oil and toast until crisp, about 8 minutes.
Meanwhile, heat the remaining 2 tablespoons olive oil in a large Dutch oven or pot over medium heat. Add the garlic and onion and cook until softened, about 5 minutes. Add the tomatoes, red pepper flakes and chicken broth and simmer, uncovered, for 10 minutes. Using a fork, lightly flake the reserved fish and add it with the oregano to the stew; simmer for 5 minutes. Season to taste with salt and pepper. Serve in bowls and top with the croutons.