Preheat oven to 190˚C.
Trim the tenderloin and spread mustard along the loin. Place in a roasting tin and pour in the apple juice and balsamic. Roast for 20-25 minutes… it may need a little longer!
When cooked, drain the apple and balsamic juice into a saucepan, add the stock and boil to reduce for 5-10 minutes until it thickens. Finally, add the butter to give the sauce a glossy finish.
Serve with sliced, griddled apples.