Heat half the oil in a large wok or non-stick frying pan over high heat. Cook half the beef for 1-2 minutes, or until browned, breaking it up as it cooks. Set aside in a large bowl. Repeat with remaining oil and beef.
Reheat wok or frypan over high heat, adding a little extra oil, if necessary.
Add onions, capsicum, carrot, mushrooms and water chestnuts and cook for 1-2 minutes. Return beef to pan with hoisin sauce and 2 tablespoons of water and cook for a further minute, tossing well to coat. Remove from heat, sprinkle with chopped coriander and bean sprouts and toss to coat.
Arrange lettuce leaves on a large platter. Spoon beef mixture into lettuce, sprinkle with extra onions, coriander, peanuts and chilli, if desired. Serve with lime wedges.
Beef stir-fry strips would also work well in this recipe.
Add thinly sliced snow peas, shredded cabbage or finely sliced ginger, if desired.