Preheat oven to 180°C.
Combine the cheese, crème fraîche and mustard in a bowl and season to taste. Spread the cheese mixture over all 8 slices of sourdough. Top 4 of the slices with the ham then place the remaining slices of sourdough on top, cheese side up.
Heat a frying pan over medium heat and add the butter. Once foaming, fry the base of the sandwiches in batches until golden and toasted. Transfer to a baking tray then place in the oven. Cook until the cheese on top is golden and bubbling. Serve immediately.