In a large resealable plastic bag, place the soy sauce, lime juice, sesame oil, Thai sweet chili sauce and garlic, shaking well to mix.
Add the flank steak; turn to coat with the marinade.
Refrigerate until ready to grill.
Coat grill rack with cooking spray.
Prepare the grill to medium-high heat.
When grill is ready, discard the marinade.
Grill steak for about 6 minutes on each side, or until they reach desired doneness.
Remove steak to cutting board and let rest about 10 minutes.
Slice steak diagonally across the grain before serving.
Into each lettuce leaf, place some seasoned, sliced steak and top with shredded carrots, bean sprouts, peanuts and extra Thai sweet chili sauce, if desired.