Preheat the oven to 400°F (200°C). Lightly grease a baking dish large enough to hold the fish and vegetables.
Arrange the cod fillets in the baking dish.
Scatter the sliced onion, garlic, cherry tomatoes, and black olives around the fish.
Drizzle the olive oil evenly over everything.
Sprinkle with Italian seasoning and season with salt and black pepper.
Pour the water into the baking dish.
Scatter the chopped butter over the fish and vegetables.
Bake the Mediterranean Baked Cod for 15–20 minutes, or until the fish flakes easily with a fork and the tomatoes are soft and juicy.
Spoon the flavorful pan juices over the fish before serving.