Heat 2 tablespoons olive oil in a large skillet over medium heat. Add ½ onion, diced, 1 tablespoon minced garlic, and 2 pounds ground chicken.
Cook until the chicken is browned and fully cooked, breaking it apart as it cooks.
Once the chicken is browned, stir in ¼ cup hoisin sauce, 3 tablespoons soy sauce, 2 tablespoons rice wine vinegar, 1 tablespoon sesame oil, 1 tablespoon sriracha sauce, and 1 can water chestnuts, drained and diced.
Reduce the heat to low and cook for 5–10 minutes, stirring occasionally, until all the ingredients are well combined and the sauce coats the chicken mixture evenly.
Separate and wash the leaves from 1 head butter lettuce. Pat dry.
Spoon the chicken mixture into the butter lettuce leaves.
Top with 3 green onions, chopped, and serve immediately.