Prep: Preheat the oven to 400°F. Lightly grease a rectangular baking dish that will comfortably hold all your cod pieces and tomatoes and place the fish inside.
Add vegetables: Scatter the onion, garlic, tomatoes and olives around the fish.
Season: Drizzle the olive oil all over the ingredients in the dish. Sprinkle with Italian seasoning and season with salt and pepper.
Finish and bake: Pour the water into the dish. Scatter the butter evenly over the dish. Bake for 15-20 minutes, or until the tomatoes and fish are cooked.