Whisk together 2 tablespoons hoisin, 1 tablespoon soy sauce, 1/2 tablespoon rice vinegar, and optional 1/2 tablespoon sriracha.
Heat 1/2 tablespoon neutral oil in a large skillet over medium-high heat. Add 1/2 pound ground chicken and cook 4 to 5 minutes, breaking it into small crumbles, until browned and 165°F.
Push the chicken aside. Add 1/2 diced onion, 1 minced garlic clove, and 1/2 tablespoon minced ginger to the open side of the skillet. Cook 2 minutes, then stir everything together.
Pour in the sauce and cook 1 to 2 minutes over medium-high heat, stirring, until it clings to the chicken rather than pooling. Stir in the chopped half-can of water chestnuts for the final minute, then fold in 1 chopped green onion.
Separate, wash, and thoroughly dry the lettuce leaves. Spoon the hot chicken mixture into the crisp leaves and serve immediately.