In a large saucepan over medium heat, gently saute onion and garlic in oil until vegetables soften, about 10 minutes. Stir all but 2 tablespoons of the parsley into the vegetables, then add the tomatoes, green pepper, pepper flakes, and 1 teaspoon salt, stirring to mix well. Cover and cook over low heat for 20 minutes.
Add water and wine and bring to a simmer. Adjust seasoning with salt and pepper. Put the fish steaks into the saucepan, covering with the sauce. Cook 15 minutes, or until fish is cooked through. Remove the fish to a plate using a slotted spoon. Set aside.
Bring sauce to a boil and reduce until it coats the back of a spoon, removing any fish bones or skin you find. Flake the cooked fish flesh and return to the reduced sauce. Cook for 5 minutes to meld flavors, then remove from heat and set aside.
Bring 6 quarts of water to a boil over high heat and add 2 tablespoons salt.
Add the spaghetti to the boiling water and cook until tender, yet al dente, about 7 to 8 minutes. Drain and add to the sauce.
Cook over high heat 45 seconds, then divide evenly among 6 heated pasta bowls, sprinkle with remaining parsley, drizzle with oil, and serve immediately.