Pre heat an oven to 180C. Season the pork and seal in a large pan.
In an oven proof dish place the onions and sage, drizzle with olive oil to coat. Place the pork on top and transfer to oven and bake for 15 minutes. Turn down the oven to 160C pour over the cider and bake for a further 50 minutes basting every 10 minutes.
Rest the pork in a loosely covered dish for 30 minutes. Keep the onion sage and cider juice for gravy. Reduce if needed.
For the mash, in a saucepan cover the potatoes in milk and simmer until cooked. Drain and mash. Peel and core the apples, quarter and cut each quarter into six chunks. Fry in butter and sugar until gently softened keeping the shape. Mix the apple and potato together, add the mustard and season.
Slice the loin in thick slices and serve on top of the mash. Spoon juice over the pork.