Combine soy sauce, sherry, mustard , ginger, thyme, and garlic in a shallow dish; stir well. Place roast in dish. Cover and marinate roast overnight in refrigerator; turn occasionally.
Remove roast to a well-greased rack in a shallow roasting pan, reserving marinade. Insert meat thermometer, at an angle, into thickest part of roast.
Bake, uncovered, at 325° for 2 hours and 45 minutes or until meat thermometer registers 170°. Baste with reserved marinade every 15 minutes during last 45 minutes of cooking time.
Remove roast to a serving platter. Garnish platter with onion chrysanthemum, cherry tomato flowers, and bay leaves, if desired. Let roast stand 10 to 15 minutes before slicing.