Place 3 tablespoons of the hoisin sauce, 2 tablespoons soy sauce, 2 tablespoons rice vinegar, and 1 teaspoon toasted sesame oil in a small bowl and whisk. If you'd like a more thickened, glossy sauce, whisk in 1 teaspoon cornstarch (optional); place near the stove.

Heat 1 teaspoon of the oil in a large skillet over medium heat until shimmering. Add 1 pound ground chicken and cook, breaking it up into small pieces, until cooked through and no longer pink, 7 to 8 minutes. Transfer the cooked chicken to a clean bowl; set aside.

Add the remaining 1 teaspoon oil to the same pan. Add 8 ounces finely chopped white button or cremini mushrooms and any optional vegetables, and cook, stirring occasionally until tender, 4 to 5 minutes.

Stir in finely chopped water chestnuts, 3 cloves minced garlic, and 1 tablespoon peeled and minced fresh ginger. Cook until fragrant, about 30 seconds.

Return the chicken to the pan and add ½ of the scallions.

Add the sauce and cook, stirring occasionally, until bubbling and the sauce is warmed through, 30 to 60 seconds.

Taste and add more hoisin sauce if desired.

Break off all the lettuce leaves and pile them on a plate in the center of the table, along with small dishes with sriracha hot sauce, red pepper flakes, and the remaining scallions. Transfer the hot chicken mixture to a serving dish, or serve straight from the skillet. Give everyone a spoon and let them dig in — place a generous spoonful of chicken mixture in the middle of a lettuce leaf, top with scallions and sriracha hot sauce or red pepper flakes, and eat right away.