Wash and separate lettuce leaves. Dry thoroughly and refrigerate until serving.
Heat oil in a skillet over medium-high heat. Add garlic, ginger, and white scallion parts. Cook for 30–60 seconds.
Add ground chicken and onion. Break apart with a spoon, cooking for 5–7 minutes until no longer pink.
Stir in water chestnuts or bell pepper. Cook for 2 minutes.
Add soy sauce, hoisin sauce, rice vinegar, sesame oil, and sriracha (if using). Stir and cook for 2–3 minutes until flavors combine.
Remove from heat, stir in scallion greens.
Spoon filling into lettuce leaves, garnish with cilantro, sesame seeds, or peanuts. Serve immediately.