In a big pot with salty boiling water, cook the peeled potatoes for about 20min. Potatoes must be soft enough to sink a knife inside without breaking them. Once ready, drain them in a sieve.
Cut the onions and sauté them in a pan with a knob of butter, first over high heat for few minutes and then low heat. Once the onions are translucent, add the white wine and let it cook until the alcohol evaporates.
In another pan, sauté the lardons. Once they are cooked, put them on a towel kitchen to absord excess fat.
On the bottom of a plate, spread half of the onions and lardons. Then add the potatoes cut in slices before spreading the other half of the onions and lardons.
Scratch the crust of the cheese before cutting it in half horizontally. Spread the cheese on top of the mixture. Season with salt and pepper before putting it in the oven for 30min at 280ºC. 3-5minutes before it's ready, put the oven on grill mode to brown it.