Prepare sauce: Whisk together 3 tablespoons soy sauce, 2 tablespoons rice vinegar, 1 tablespoons sesame oil, and 1 tablespoons fresh ginger, minced. Set aside.
Sear chicken: Heat 1 tablespoons neutral oil for cooking in a large skillet over medium-high heat. Add 1.5 pounds ground chicken and cook until golden, breaking it apart as it cooks, about 5-6 minutes. Drain excess fat if needed.
Build flavor: Add 3 garlic cloves, minced to the pan and cook for 30 seconds until fragrant. Stir in 0.5 cups water chestnuts, diced and 0.5 teaspoons red pepper flakes.
Sauce and finish: Pour the sauce over the chicken mixture. Toss well and cook for 1 minute to coat everything. Remove from heat and stir in 4 green onions, chopped and 0.3 cups fresh cilantro.
Assemble: Spoon filling into lettuce leaves and serve immediately. Add extra 0.3 cups fresh cilantro and 4 green onions, chopped on top if desired.