Preheat your oven to 200°C (390°F).
Peel and slice the potatoes into thin rounds.
In a pot, boil the potato slices for about 10 minutes until slightly tender.
Drain the potatoes and set aside.
In a skillet, cook the lardons or bacon until crispy; remove and set aside.
In the same skillet, sauté the chopped onion until translucent.
In a large baking dish, layer half of the potatoes, followed by half of the bacon, and half of the sautéed onions.
Spread half of the crème fraîche over the layers.
Repeat the layers with the remaining ingredients.
Top with sliced reblochon cheese and bake for 25-30 minutes until golden and bubbly.