Mix all the ingredients for the sauce in a small bowl.
Heat 1 tablespoon oil in a large skillet over medium high heat. When the oil is hot, add ground chicken. Chop and stir, until separated to smaller pieces and the surface turns light brown but the inside is still a bit pink. Transfer the chicken to a plate and set aside. If your chicken sticks to the pan, pour in a tablespoon of cooking wine (dry sherry, Shaoxing wine, etc.) or chicken stock to deglaze, and transfer the brown bits to the plate with the chicken.
Add the rest – 1 tablespoon oil, onions and mushrooms. Cook until the onions turn tender and the mushrooms are browned.
Add the garlic and ginger. Cook for a few seconds to release the aroma.
Add back the cooked chicken and water chestnuts. Pour in sauce. Stir to mix well, until the sauce is mostly absorbed. Carefully taste the chicken. Adjust the seasoning by adding salt, if necessary. I prefer to not season too much during this stage because you might use other sauces to serve the dish.
Transfer the chicken onto a big plate and garnish with the green part of green onions and sesame seeds, if you’re using them.
Serve warm with separated lettuce leaves, chili oil (or any other hot sauce you prefer), hoisin sauce, and chopped nuts on the side.
To eat, spoon the chicken mixture into the center of a lettuce leaf and add any preferred toppings. Enjoy it tacostyle!