Season both sides of the mackerel with plenty of salt and pepper and coat it in a thin layer of flour (or cornstarch).
Heat some olive oil up in a frying pan, add the mackerel, and cook until both sides are browned and crispy.
Slice the onion and tomato into rings and lightly toast the bread in an oven.
Cut the bread in half and make a deep horizontal slice in each piece. (Don't cut all the way through.)
Open up the bread and add the mackerel, onion, and tomato. Sprinkle plenty of lemon juice over the top.
Your sandwich is done! Try adding in a little Italian parsley too if you feel like it.
Story Behind this Recipe
When I was studying abroad in Turkey, I often ate this when I missed Japanese food. I always ate it while looking out over the Bosphorus Strait and feeling the salty sea breeze. I never had to make it myself while I was there, I would just buy it from the stands along the beach. You should use the most delicious batard bread that you can get your hands on.
Add tons of onion to get the authentic Istanbul flavour! (I didn't add so much in the photo to make it look nicer.) There should be enough so that you can really taste the kick of the onion. If you can't eat the sandwich straight away, brush the inside of the bread with olive oil to prevent it from going soggy.