In a large skillet, heat the vegetable oil over medium-high heat. Add the ground beef and cook until browned, breaking it apart with a spatula, about 5-7 minutes. Drain excess fat if necessary.

Add the minced garlic and ginger to the skillet, stirring for about 1 minute until fragrant.

Stir in the diced red bell pepper and water chestnuts, cooking for an additional 3-4 minutes until the vegetables are tender.

In a small bowl, mix together the soy sauce, hoisin sauce, rice vinegar, sesame oil, and red pepper flakes (if using). Pour this sauce over the beef mixture and stir to combine. Cook for another 2-3 minutes until heated through.

To serve, spoon the beef mixture into the center of each lettuce leaf. Top with sliced green onions and sesame seeds if desired.