Heat a large frying pan over medium heat until hot, Sear roast, fat side down, then on all sides, Place in a roasting pan.
Make a paste with butter, flour, mustards, peppercorns, sugar and thyme Spread over top and sides of the roast. (Allow the meat to cool slightly, before adding the peppercorn paste. It makes it easier to apply.)
Roast, uncovered for 1 hour and 15 minutes, or until meat thermometer reads 160, and juices run clear.
Place the roast on a cutting board.
Skim most of fat from pan and discard Pour the juices into a medium saucepan, whisk in flour, cook over medium high heat 2 minutes, whisk in cider, vinegar, sugar, mustard and salt, simmer 5 minutes or until thickened.