Peel potatoes and cut into thick slices, then boil in water with a pinch of salt until cooked.
Drain potatoes and let cool. The slices should be cooked, yet still retain their shape.
Add some olive oil to a frying pan and Sauté the chopped onions. Then add the diced bacon and finally the sliced mushrooms, when it's almost finished.
Toss the potatoes with some ground black pepper. Then prepare a buttered gratin dish.
Add a layer of potatoes to cover the bottom of the gratin dish, using about half of the potatoes.
Then add the onions, mushrooms and bacon as a second layer.
Add the remaining potatoes on top of this, plus the optional half a glass of wine, then cover the lot with the double cream.
Slice the reblochon cheese in half (I used fromage roux as a substitute in Japan) and place this on top.
Finally bake in an oven at 250°C or 480°F, until the cheese melts and browns on the surface.