01 - Whisk together hoisin sauce, soy sauce, oyster sauce, rice vinegar, sesame oil, sriracha if using, and brown sugar in a small bowl until well combined. Set aside.
02 - Heat vegetable oil in a large skillet or wok over medium-high heat. Add diced onion and cook for 2 minutes until softened and translucent.
03 - Stir in minced garlic and grated ginger. Cook for 30 seconds until fragrant, being careful not to burn the garlic.
04 - Add ground chicken to the skillet, breaking it up with a spatula. Cook for 4 to 5 minutes until fully browned and no longer pink.
05 - Add chopped mushrooms and continue cooking for 2 minutes until softened. Stir in diced water chestnuts and sliced green onions; cook for 1 minute.
06 - Pour the prepared sauce over the chicken mixture, stirring constantly to coat evenly. Cook for 2 to 3 minutes until the sauce is absorbed and the filling appears glossy.
07 - Remove from heat. Spoon warm filling into individual lettuce leaves. Garnish with additional sliced green onions and sesame seeds as desired. Serve immediately while warm.