Peel and slice the potatoes very thinly (a mandolin works best to get them very thin).
Peel and slice the onions medium-fine (again the madolin, if you have one).
Heat the oven to 350 degrees.
Melt the butter and pork fat, (you can replace the pork fat with butter, but the taste will be different), in a large iron frying pan over a medium low heat and add the onions, cooking them for 4 to 5 minutes until transparent before adding the potatoes.
Season well with salt and pepper.
Turn them over occasionally with a wooden spatula and when they have coloured to the blond stage, mix in the parsley.
Empty the contents of the pan into a large oval gratin dish and cook very slowly in the oven for at least 1 hour or until browned and crusted on the surface and melting underneath.