Clean the fish, don’t remove head, keep it whole. Put some rosemary leaves inside the fish. Peel potato and cut like chips, but about 0,5 to 1 cm thick. Peel onion and garlic. Cut the onion into halves and garlic into 4 pieces. Cut tomato into halves. First lay potato into the casserole pot and than lay fish. Place onion, tomato and olives around fish. Season with salt and pepper. Pour wine over everything, and than olive oil. Place rosemary branch between fish and sprinkle with oregano. Preheat oven to 200°C and put fish to bake about 30 - 40 minutes. You can reduce heat to 180°C in the middle of baking, than heat again 5 minutes before end to 200°C. If you are not sure potato will bake you can cover the casserole pot with aluminum foil, but remove it at least 15 minutes before end to prevent “cooking” the fish.