Heat the neutral oil in a wide skillet over medium-high heat and cook the chicken for 6 minutes, breaking it into small crumbles.
Add the mushrooms and water chestnuts and cook for 5 minutes until their released liquid evaporates and the chicken reaches 165°F.
Stir in the hoisin, soy sauce, rice vinegar and sesame oil and cook for 1 minute until the filling looks glossy, not wet.
Fold in the scallions and remove the skillet from the heat.
Spoon the warm filling into the 12 dry lettuce leaves and serve immediately.