Heat a large frying pan over medium-high. Add oil, then chicken. Stir often with a fork to keep meat crumbly, cooking until meat is no longer pink, 2 to 3 min. Add mushrooms and water chestnuts. Cook, stirring often, for 2 min.
Stir broth with soy, sugar and cornstarch in a medium bowl until cornstarch dissolves. Add to chicken mixture. Cook until sauce is slightly thickened, about 2 min. Remove from heat. Stir in carrot and green onions. Season with pepper. Scoop a heaping 1/3 cup of filling onto each lettuce leaf. Serve with hot chili sauce if desired.
Nutrition (per serving)
Serving Tip: No lettuce? Roll filling into a whole-wheat tortilla, or serve over rice.