Hosios David, Thessaloniki. 5th C church and mosaic, 12th C frescoes. The theophany - apocalyptic appearance of the Lord - is really unusual. I love the beardless Jesus, when he shows up. There are a number of miracle legends associated with the mosaic. My favorite is that in the late days of Iconoclasm, an Egyptian monk was praying in the monastery when with a great shake of the earth, the mosaic appeared! He took it as a validation of icons and as a representation of the Real Jesus. I teach this one all the time.