Thaw fish if frozen. Rinse fish and pat dry with paper towels. Preheat oven to 400 F degrees. Grease a baking dish with cooking spray.
In the prepared baking dish combine tomatoes, onions, oregano, oil, garlic, 1/4 tsp of the salt, and 1/4 tsp of the pepper. Toss to coat the tomatoes.
Roast, uncovered, for 15 minutes. Place fish, skin side down, on top of the tomato-onion mixture. Sprinkle fish with remaining 1/4 tsp pepper and 1/4 tsp salt. Roast, uncovered, for another 15 to 20 minutes or until fish begins to flake easily when tested with a fork.