Peel and wash the potatoes. Place them in a pan filled with water. Add a pinch of salt and bring to a boil.
In the mean time, preheat the oven to 180°C (350°F).
Peel and chop the onions. Dice the bacon.
Sauté the onions and bacon in a pan.
When the potatoes are cooked, cut them into fairly thick slices.
Arrange the potatoes, onions and bacon in a gratin dish. Season with salt and pepper.
Add a little crème fraîche and mix gently, making sure not to break the potatoes.
Cut the Reblochon in half across the top, then in half again along the side. Place the pieces of Reblochon on top of the potato mixture.
Bake for 20 minutes, until the top is nice and golden-brown.
