Wash potatoes thoroughly and cook in boiling water for 18-20 minutes. Drain, rinse and peel. Cut potatoes into 3-4 mm thick slices. Peel onions and cut into fine rings.
Finely dice the bacon. Heat butter in a large pan. Sauté onions in it over a low heat until translucent. Raise the temperature and lightly brown the onions while turning them. Add bacon and fry over medium heat for another 4-5 minutes.
Add potato slices, mix well and fry for another 4 minutes. Douse with white wine, season with pepper and let it simmer a little bit. Stir in crème fraîche and season with salt. Put the potato mixture into an ovenproof dish.
Cut the cheese in half horizontally. Place the halves next to each other with the rind facing down on the potato mixture. Cover the mould with aluminium foil. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 20 minutes.
The cheese should have melted well by now. Take it out of the oven, remove foil. Mix well and bake at the same temperature for another 15-20 minutes.