In a small bowl, whisk together the hoisin sauce, soy sauce, rice vinegar, honey, and sriracha (if using). Add the cornstarch and whisk until completely smooth and no lumps remain. Set aside.

Carefully separate the leaves from the head of lettuce, keeping them whole to form cups. Wash and gently pat them dry with a paper towel. Arrange the lettuce cups on a serving platter.

Heat the sesame oil in a large skillet or wok over medium-high heat. Add the ground chicken and cook, breaking it up with a spoon, for 5-7 minutes until it's browned and fully cooked. Drain any excess grease from the pan.

Add the finely chopped yellow onion to the skillet with the chicken. Cook for 3-4 minutes, stirring occasionally, until the onion has softened.

Stir in the minced garlic, grated ginger, and the white parts of the sliced green onions. Cook for 1 minute more until fragrant.

Give the prepared sauce a quick stir, then pour it over the chicken mixture. Stir everything to combine and bring the mixture to a simmer. Let it cook for 2-3 minutes, stirring frequently, as the sauce thickens and glazes the chicken.

Remove the skillet from the heat. Stir in the chopped water chestnuts and the green parts of the green onions.

Spoon the warm chicken filling into the prepared lettuce cups. Serve immediately, garnished with extra green onions or sesame seeds if desired.