Prepare the Hoisin Sauce by combining minced garlic, grated ginger, hoisin sauce, soy sauce, rice vinegar, chili paste, and ground white pepper in a bowl. Set the sauce aside.
Separate the leaves of the romaine or butter lettuce and thoroughly wash and dry them.
Heat olive oil in a pan over medium heat, then add ground chicken. Crumble the chicken and cook for about 2 minutes.
Add the diced yellow onion to the pan and continue cooking until the chicken is no longer pink and the onions are soft, about 5 minutes. Drain any excess fat from the pan.
Stir in the chopped water chestnuts. Cook for about a minute.
Pour 3/4 of the prepared Hoisin Sauce into the pan with the chicken mixture. Cook for an additional 2-3 minutes, or until the sauce thickens and coats the ingredients.
To assemble, take individual lettuce leaves and fill each one with about 1/4 cup of the chicken filling.
Top each filled lettuce cup with extra Hoisin Sauce and sliced green onions.