Preheat oven to 325 degrees.  Combine first 5 ingredients in a bowl and rub into meat.  Combine sherry, ketchup, and brown sugar in a bowl.  Place meat in an oven bag with potato starch.  Pour sauce over meat and add onion.  Put bag in roasting pan.  Make 6 small cuts in bag, according to package directions.  Roast 2 1/2 to 3 hours, or until tender.  Place carrots in a steamer basket over boiling water.  Cover pan and steam about 4 minutes, until just tender.  Remove roast from oven.  Strain juices from roasting bag and serve as gravy.  Serve with steamed carrots.