At the north end of the Galata Bridge, and just before the start of the seafood market, stands a small kiosk where a man grills mackerel filets for sandwiches.
A few shakes of spices, some salad, a squeeze of lemon, and you're on your way to the best fish sandwich in Istanbul.
If you're lucky, or if you just look pitiful standing in the pouring rain at night to get one more sandwich before you leave, you might get him to bring out some cold purple juice that he swears goes well with fish. Don't go thinking this is pomegranate juice. The pickled purple carrot juice is like a blast of firey salt water, but after the initial shock, it does sort of work with the fish. But maybe not as well as pomegranate juice would.