Hello Tilly,
This is a first for me,as I've never come across anyone making dauphinoise with creme fraiche before.
Take my advice,love,and never fret over this again....
Maris piper pots,cut into 1/8th slices,briefly rinsed in cold running water,leave to drain.
Arrange pots in an oven proof container,(cover with damp cloth to prevent them going black.
Mix cream and milk (70/30)ratio,white ground pepper,and a lot of salt.Remove cloth,and pour mixture over pots.
Agitate a little to ensure the liquid has settled and covered all the pots.Cover with a double layer of tin foil,make sure the foil does not touch the pots,or where it does,the pots will be grey/black.
Do not puncture the foil,place in a pre heated oven(120-140),for about 90-105 mins.If you want to brown the top before you serve,whack the oven as high as it will go,remove the foil,place the pots to the upper most part ,about 5-10 mins should suffice.
Serve to dizzying applause and un-ending admiration !!!
In a previous life,I was a professional chef for 35 years.(just to prove that I actually DO know what I'm talking about).
And please,don't ever cook dauph's from scratch in a microwave.