In a medium bowl, whisk together soy sauce, sugar, sesame oil, minced ginger, cayenne pepper, and crushed garlic until well blended.

Add beef cubes to the marinade, ensuring all pieces are evenly coated.

Set the marinated beef aside to allow flavors to meld.

Heat a nonstick 12-inch skillet over medium-high heat.

Add celery, carrots, and a splash of water to the skillet and bring to a boil.

Cook, stirring occasionally, until the vegetables are tender-crisp (approximately 2-3 minutes).

Add the marinated beef and the remaining marinade to the skillet.

Cook, stir-frying constantly, until the beef just loses its pink color (about 2 minutes).

Stir in green onions and sesame seeds, and cook for another minute, stirring continuously.

To serve, place a few tablespoons of the beef mixture onto a fresh lettuce leaf.

Wrap the lettuce leaf around the beef mixture and eat immediately.