Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the ground beef and onion, cooking until the beef is thoroughly browned and the onion is translucent, breaking up the meat into small crumbles as it cooks.
Stir in the garlic and ginger, cooking for 60 seconds until fragrant.
Whisk together the soy sauce, hoisin sauce, rice vinegar, and sesame oil in a small bowl.
Pour the sauce mixture into the skillet and add the chopped water chestnuts.
Simmer for 2 to 3 minutes, stirring constantly until the sauce coats the beef evenly.
Remove from heat and stir in half of the sliced green onions.
Spoon the beef mixture into the center of the prepared lettuce leaves.
Garnish with the remaining green onions and serve immediately.