Boil the potato for 20 mins, drain and leave to cool. Fry the bacon lardons in a non-stick pan until crisp, then drain on kitchen paper.
Take the slices of sourdough bread and spread one with the crème fraîche, season and sprinkle with caraway seeds. Top with the grated Gruyère and the lardons. Slice the potato over the top and cover with the sauerkraut, sliced Reblochon or Camembert, then close the sandwich with the other slice of bread.
Fry the sandwich in the sunflower oil and butter for 3-4 mins on each side. Serve warm.