Place the slices of pork fillet onto a board and use a meat mallet or rolling pin to flatten them out into thin slices. Season with salt and freshly ground black pepper.
Heat a frying pan until hot, add half the butter and the slices of pork fillet and fry on each side for 1-2 minutes, or until golden and just cooked through (you may need to do this in two or three batches to avoid overcrowding the pan). Transfer to a warm serving plate and keep warm.
Add the Armagnac to the pan used to fry the pork fillet. Carefully flambée the Armagnac then add the prunes and cook for two minutes, mashing the prunes lightly with the back of a spoon or spatula.
Add the cream, season with salt and freshly ground black pepper, and cook for a further minute.
Heat a separate pan until hot and add the remaining butter, olive oil and potatoes. Sauté the potatoes for 4-5 minutes, or until golden-brown and tender.
Add the shallot to the potato and fry for a further minute then stir in the flatleaf parsley and season, to taste, with salt and freshly ground black pepper.
To serve, spoon the sauce over the pork fillet and pile the potatoes alongside.