Preheat oven to 300°F (150°C). Season fish fillet with salt and pepper.
Spread onion on a small heatproof dish and place fish fillet on top.
Spray some oil on top and cover with tomatoes slices. Cover with mozzarella cheese.
Sprinkle dry oregano on top and bake in the oven until fish is just cooked.
Combine arugula leaves with lettuce and season with salt and balsamic vinegar.
Arrange arugula and lettuce salad on a dish and place fish on top.
How to fillet, skin and debone fish