Preheat the oven to 350 degrees F (175 degrees C). Coat the bottom of a baking dish with olive oil.

Sprinkle 1/2 the onion and garlic evenly in the baking dish, and cover with 1/2 the pasta sauce. Place haddock fillets in the dish, top with tomatoes and remaining onion and garlic. Cover with remaining pasta sauce.

Bake 20 minutes in the preheated oven. Top with mozzarella cheese, and continue baking 10 minutes, until cheese is melted and fish is easily flaked with a fork.