Wash halibut and pat dry. Cut into serving size pieces and place in a 20x30cm baking dish. Sprinkle with salt and pepper. Stir together the olive oil, parsley, onion, celery, pepper, tomatoes, capers and garlic; pour over the halibut.
Bake until halibut is slightly opaque in the centre, about 20 minutes. Remove from oven; let stand for 10 minutes before serving.