I have Bing set as my homepage search engine, and I was wondering if this is where Joan is doing her nun learning time? To me, it would make sense for her to go, and where Jamie could visit her. https://en.wikipedia.org/wiki/Mont-Saint-Michel
I don't know if it's said in the books, because I haven't read any of them, but it would fit the time period. I know that Claire was a World War 2 English nurse, so I wonder if she spent time in this area treating wounded solders on ships or on the island. All I know is that Claire, in the show, is shown how V-E day was celebrated, but I don't know if it's really said what area of Europe she's stationed at when this starts.