Okay here is the deal, Istanbul is full of mosques and churches. Some are very easily reachable while some are hard because even the locals are not aware of them. This one is called St. Mary Pammakaristos Church or The Church Of The Theotokos Pammakaristos (The Joyous Mother Of God) originally, strangely it was not immediately converted into a mosque after the conquest of Constantinople but operated as a Church until 1573.
It’s in a district which does reflect modern Turkey one bit but the experience is fabulous. Honestly as a local when I go to this district I feel like a foreigner. This unique Byzantine Church is standing on the terrace overlooking the Golden Horn. It is located in a rather open field where you can walk around and observe the structure. The mosaics of the church are not as mind-blowing as St. Chora but yet are unique in their context. You can see the naked mosaic of Jesus Christ which is a rare scene. Unfortunately most of the interior walls were demolished but then restored when the structure was converted into a museum. You can visit this off the beaten church/mosque/museum if you are in the area. Don’t forget to take photos as the mosaics are losing its touch as years go by.