Mix all ingredients for marinade. Let meat marinate overnight in the mixture, then remove meat, dry thoroughly, and reserve marinade for au jus. Roast the fillet in a 400 degree oven until the temperature reaches 125 degrees. This will take approximately half hour. Remove from oven and let rest, covered with foil, for half hour. Rosemary Au Jus Saute the onions until well browned, about 15 minutes. Add the rosemary, sherry, and marinade, and bring to a boil. Add the stock, bring to a boil, thicken with the corn starch mixture, cook for 1 minute. Serve over fillet.