Make the special sauce-sugar sauce by dissolving sugar in water in a small bowl. Add the soy sauce, mirin, lemon juice, sesame oil, mustard, chili garlic sauce and water. (See notes below: you can also add ketchup to the sauce but I never do)
Whisk and refrigerate sauce until ready to serve
Bring oil to high heat in a skillet. Saute ground chicken 4-5 minutes. Break up the chicken into small pieces.
Add a bit more oil, water chestnuts, mushrooms and garlic to chicken
Prepare the stir fry sauce in a bowl by mixing the soy sauce, Worcestershire sauce and mirin
Add stir fry sauce to the pan and saute mixture a couple of minutes. Remove the special sauce you set aside earlier and add a 2- 3 Tbsp of that sauce to the chicken mixture.
Transfer chicken mixture to a serving platter.
Serve chicken mixture in lettuce leaves and spoon special sauce on top