In a large skillet, heat the vegetable oil over medium heat. Add the chopped onion and sauté for about 3-4 minutes until softened.
Stir in the minced garlic and ginger, cooking for an additional 1-2 minutes until fragrant.
Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned and cooked through, about 5-7 minutes. Drain excess fat if necessary.
Stir in the soy sauce, hoisin sauce, rice vinegar, and sesame oil. Mix well and cook for another 2-3 minutes.
Add the chopped water chestnuts and green onions, stirring to combine. Cook for an additional minute to heat through.
To serve, spoon the beef mixture into individual lettuce leaves. Garnish with sesame seeds if desired.