Put the potatoes in a large pan of cold water and bring to the boil. Simmer for 20 minutes until the potatoes are very tender. Remove from the heat, drain and leave in the colander over the pan for a couple of minutes for the moisture to evaporate.
Return to the pan, add the milk and butter and mash until smooth. Stir through the apple sauce. Keep warm until required.
Meanwhile, heat 1 tbsp olive oil in a large frying pan and fry the onion for 5 minutes, until softened. Add the garlic and cook for a further 1 minute, then add the sliced mushrooms. Cook for 3-4 minutes until the mushrooms are golden, then add the stock, crème fraîche and mustard and bring to a simmer. Mix the cornflour with 1 tsp water and add to the sauce. Simmer until the sauce thickens. Season with black pepper.
While the sauce is thickening, heat the ½ tbsp oil in a pan. Fry the pork loins for 10-12 minutes or until cooked through and no pink remains, turning halfway through the cooking time.
Plate the pork with the apple mash and spoon over some of the sauce. Serve with the green beans on the side.