Make the special sauce-sugar sauce by dissolving sugar in water in a small bowl. Add the soy sauce, mirin, ketchup, lemon juice and sesame oil
Mix well and refrigerate sauce until ready to serve
In a separate bowl, for the mustard sauce combine hot water, mustard and gochujang – whisk this and set aside
Bring oil to high heat in a skillet. Saute ground chicken and onions for 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 and transfer chicken mixture to a serving platter.
Remove two sauces you set aside earlier (sugar sauce/mustard sauce) and whisk them together
Serve chicken mixture in lettuce leaves and spoon mixed sauce on top or serve on side for dipping