First par boil some fairly large potatoes in salted water - traditionally it seems to be red potatoes, but any are fine. Leave to cool.
Soften some chopped onion in oil or butter and scatter with finely chopped thyme.
Transfer to the bottom of an oven proof dish.
Arrange a layer of bacon rashers over the onions.
When the potatoes are cool enough rub off the skins and slice about 1/2 a cm thick.
Arrange the sliced potatoes on top of the bacon and season.
Dot with small pieces of butter.
On the very top its traditional to place a Reblochon cheese with the bottom and side skin trimmed off (any soft and runny cheese will do though).
Place in an oven at 180 degrees and cook until the potatoes are cooked through and the chesse has melted into and across the dish.
Serve with seasonal green veg.