Mosaics around Palermo

I remember somewhere in Rick's videos or books that he compared the Byzantine-style mosaics in Cefalu, Palermo and Monreale. He talked about putting the pantocrator onto the apse, a curved space and how the artists were learning how to use that whole space (rather than the flat dome at the top of a church). Does anyone remember where that was? I want to quote it in a paper I am writing.