This is an impressive and delectable dish, best made with large, succulent, fresh scallops. Ratte potatoes are an old variety found in France, also called Asparges. They're small 'finger' like potatoes with a nutty flavour. If you can't find them, use any small, waxy potatoes, which will also work a treat.
A very nice dish, a bit fiddly but worth the effort. I didn't help myself by adding a giant king prawn to the side of the dish though. Many thanks for a lovely meal - 14 Aug 2015