In a small bowl, combine the soy sauce, oyster sauce, 1 teaspoon sesame oil, sugar, & black pepper. Pour on top of the ground chicken, stir to combine. Set aside for 15 minutes to marinade.
Meanwhile, make your drizzling sauce. Combine 4 heaping tablespoons of hoisin sauce, 1 teaspoon of chili sauce, & 1 tablespoon warm water. Set aside.
Heat 1.5 teaspoons of sesame oil in a skillet. Add in the minced garlic & cook for 30 seconds. Add the marinaded chicken to the skillet along with the diced water chestnuts. Break up the chicken with a wooden spatula, and cook on medium-high heat until the chicken is browned and cooked through.
Scoop your desired amount of ground chicken into the your lettuce leaves. Drizzle with a little bit of the sauce, & top with chopped green onions.