Coat a medium skillet with cooking spray. Cook bell pepper and onion over low heat for 5 minutes. Remove vegetables from skillet and set aside.
Cook the chicken and garlic powder in the skillet over medium heat. Use a wooden spoon to break up the chicken. When it is no longer pink, drain the fat from the skillet and add the vegetables.
Add the hoisin sauce, soy sauce, siracha, and rice. Cook for 5-8 minutes or until everything is thoroughly heated and the liquid is absorbed.
Stir in the water chestnuts.
Serve in lettuce leaves.