In a small bowl, whisk together the sauce until combined.
Heat the oil in a non-stick skillet and place over medium heat. Once hot, add the chicken, break it apart, and stir fry for 2-3 minutes until no longer pink.
Push the chicken to one side of the pan, add the chestnuts, bell pepper, garlic and onion and stir fry for several minutes.
Add the sauce and cook everything together for 1-2 minutes, until everything is evenly coated.
Place the filling in the lettuce cups and serve immediately.
Notes
TO STORE: Keep the filling and the lettuce separate and store both in airtight containers in the refrigerator. The filling should keep for 3-4 days, and the lettuce should keep for 2-3 days. TO FREEZE: You can freeze the chicken filling if desired. Just allow it to cool completely before transferring it to an airtight container or freezer-safe bag. Freeze the chicken for up to 3 months. TO REHEAT: Heat a skillet or wok to medium heat and add oil. When the pan is hot, add the filling and cook until fully reheated.