To prepare sauce: Combine oyster sauce, water, hoisin sauce, vinegar, sherry (or rice wine), cornstarch, brown sugar, soy sauce and sesame oil in a small bowl.
To prepare stir-fry: Heat 2 teaspoons canola oil over medium-high heat in a large nonstick skillet or wok. Add pork; cook, stirring constantly, until no longer pink, about 4 minutes. Transfer to a plate. Wipe out the pan.dd remaining 1 teaspoon oil, garlic and ginger; cook, stirring constantly, until fragrant, 30 seconds. Add water chestnuts, bamboo shoots and mushrooms; cook, stirring often, until the mushrooms have softened, about 4 minutes. Return the pork to the pan and add the sauce. Cook, stirring constantly, until a thick glossy sauce has formed, about 1 minute. Serve sprinkled with scallions and wrapped in lettuce leaves.