Preheat the oven to 180°C (350°F).
Place the fish fillets in a baking dish.
In a bowl, mix together the chopped tomatoes, sliced onion, minced garlic, olive oil, lemon juice, dried oregano, salt, and pepper.
Pour the tomato and onion mixture over the fish fillets and spread it evenly.
Bake in the preheated oven for 25 minutes, or until the fish is cooked through and flakes easily with a fork.
Serve the baked fish with tomato and onion sauce hot, garnished with fresh herbs if desired.