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 1/2 hour. Remove from oven and let rest, covered with foil, for 1/2 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.