Heat the oil over a moderate heat in a wok or large frying pan, add the garlic and cook for 30 seconds, or until fragrant. Add the mince, breaking up any lumps with a wooden spatula, and cook for 5–6 minutes, or until the mince browns slightly. Meanwhile, combine the oyster sauce, soy sauce and sugar in a small bowl.
Add the water chestnuts, spring onions and oyster sauce mixture to the wok. Toss for 1 minute before adding the cornflour mixture to thicken.
If you like, trim the lettuce leaves with kitchen scissors to neaten them and to form a cup shape. Divide the mince mixture among the lettuce cups, then drizzle each one with a little hoisin sauce. Eat while hot.