Turn on the oven to heat up to 200 ° C. Grease a baking dish with vegetable oil. Cut the peeled onion into thin half-rings and place it on the bottom of a baking dish.
Cut the fish fillet into portions, season with salt, pepper and put on the onion.
Put chopped tomatoes and halves of olives on top of the fish, season with salt, pepper, sprinkle with dried basil, drizzle with olive oil. Bake for 20 minutes in an oven heated to 200 °C.