Brown the roast on all sides, along with the oil and the onions in a casserole.
Mix the mustard and the fresh cream together, and baste the roast.
Cut the potatoes into large cubes and place around the roast.
Cover all with the cider and leave to simmer, with the lid on; for about 2 hours over a very low heat.
A half an hour before serving, turn up the heat and remove the lid to reduce the sauce.