Heat a wok over high heat until hot. Add 2 teaspoons oil and the mushrooms. Stir-fry for 3 minutes or until golden. Remove to a large bowl.
Add 2 teaspoons oil and swirl to coat the wok surface. Add half the mince, lemongrass and chilli. Stir-fry, breaking up any lumps, for 3–4 minutes or until pork changes colour. Add to the mushrooms. Repeat with the remaining oil, pork, lemongrass and chilli. Stand for 15 minutes to cool slightly.
Whisk ¼ cup lime juice, fish sauce, sweet chilli sauce and sugar in a jug until well combined. Spoon the dressing over the warm pork and mushrooms. Add the green onions, coriander and mint leaves. Stir gently to combine.
Arrange the lettuce leaves on a platter. Spoon over the pork mixture and sprinkle with peanuts. Serve.