Place the potatoes and apple in a clean tea towel and squeeze out as much water as possible.
Tip into a bowl and add the cheese, ham, creme fraiche, Dijon mustard, egg yolk, chives, salt and pepper.
Mix thoroughly.
Heat a large non-stick frying pan over a low-medium heat, and add a good trickle of oil.
Divide the rosti mixture into 6 and spoon into the frying pan (doing in a couple of batches of your pan isn't big enough), pressing down with the back of the spoon to give a flat surface.
Cook for about 5 minutes before carefully turning over and continuing to cook for a further 5 minutes, until both sides are golden and crisp.