Place mushrooms in a small heatproof bowl. Add enough boiling water to cover. Set aside for 30 minutes, until softened. Drain mushrooms and thinly slice.
Meanwhile, place lettuce leaves in a large bowl of iced water. Refrigerate for 30 minutes or until crisp.
Heat oil in a wok or large frying pan over medium heat. Cook onion, stirring, until fragrant. Add chicken and increase heat to high. Cook, stirring with a wooden spoon, to break up any lumps, for 5 minutes, until browned.
Add mushrooms, soy sauce, oyster sauce, sesame oil and 1/4 cup water. Reduce heat to low and simmer, stirring occasionally, for 10 minutes or until slightly thickened.
Drain lettuce leaves and pat dry. Place on serving plates and fill with chicken mixture. Top with bean sprouts and garlic chives.