Preheat oven to 425 degrees F.
Slice onion and spread onion over bottom of a shallow baking dish.
Arrange fish fillets over onion in dish, overlapping thin edges.
In a small bowl, whisk together olive oil, cooking wine, minced garlic, lemon juice, milk and oregano; pour over fish. Sprinkle with salt and pepper.
Cover with foil and bake at 425 degrees for about 10 minutes.
Uncover; sprinkle with cheddar, Parmesan and breadcrumbs.
Bake, uncovered, for another 10 minutes or until top is golden brown, cheese is melted and fish flakes easily.
Slice cherry tomatoes in half; toss with lettuce and dressing. Serve as a side to the fish, along with any leftover fruit you have on hand.