Cut steak in very thin slivers and place in a bowl.
Add soy sauce, sherry, ginger, garlic, Five Spice Powder and refrigerate 1 hour, stirring occasionally. Heat oil in a skillet or wok. Add onions and red pepper; stir-fry 1 minute.
Add beef mixture to onion mixture and stir-fry 2-3 minutes. In a custard cup, blend cornstarch and water until smooth; add to beef mixture. Cook 1 minute, stirring constantly. Spoon beef mixture into lettuce cups.
Garnish with parsley sprigs, if desired.