Boil the potatoes under a watchful eye - until a sharp knife just about slips through a cube. While you drain the potatoes, have a glug of olive oil heating in a frying pan. Pop the potatoes into the pan; toss gently every few minutes, adding the onion and apples about 10 minutes in. Stir in mustard just before serving. Great with pork chops - not forgetting a glass of crisp cider.