Wash the potatoes and cook covered in water for about 20 minutes. In the meantime peel onions and cut them into thin rings. Dice bacon finely.
Drain the potatoes, rinse and peel. Cut potatoes into slices. Heat butter in a flat roasting pan. Fry bacon cubes in it until crispy. Add onions and fry until golden brown, turning.
Mix in the potatoes and fry for about 5 minutes, turning. Season with salt and pepper. Add wine and simmer for 5 minutes. Stir in crème fraîche.
Spread cheese slices evenly on the potato mixture. Cover the roaster with a lid or aluminium foil and bake in a preheated oven (electric oven: 175 °C/circulating air: 150 °C/gas: see manufacturer) for 20-25 minutes until the cheese has melted.
Then mix the melted cheese into the potatoes and bake the casserole open at the same temperature for another 15-18 minutes until golden brown.