Cut beef across the grain in thin slices; combine lemon peel, juice, 2 tablespoons sugar, sesame oil, 1 teaspoon salt and black pepper in medium bowl; add beef; toss to coat; cover and refrigerate at least 1 hour.
Combine water, vinegar, remaining 4 tablespoons sugar and 1/4 teaspoon salt in small saucepan; bring to a boil; boil 5 minutes without stirring until syrupy; stir in crushed red pepper; set aside.
Remove beef from marinade; discard marinade.
Thread beef onto metal or wooden skewers (soak wooden skewers in hot water 30 minutes to prevent burning); grill beef over medium-hot briquette about 3 minutes per side until cooked through.
Grill tortillas until hot.
Place lettuce, onion, beef, mint and cilantro on tortillas; drizzle with vinegar mixture; roll tortillas to enclose filling.
Note: You can also grill these under a preheated broiler or flash-fry.