Heat a medium frying pan over high heat until hot. Add the oil and mushrooms and cook, stirring occasionally, for 5 minutes or until the mushrooms are tender. Transfer to a plate lined with paper towel and set aside to cool 15 minutes.
Place the duck in a food processor with the peanuts and green onions and pulse until finely chopped, transfer to a bowl. Drain any excess moister from the mushrooms and process until very finely chopped, add to the duck mixture. Add hoisin and mix until well combined.
Brush half the gow gee lightly with water. Place 1 heaped teaspoon of mushroom duck mixture into the centre of one gow gee then fold in half to form a semicircle. Pinch edges together to seal.
Half fill a large frying pan or wok with hot water & bring to the boil over high heat. Line a large bamboo steamer with baking paper and arrange the gow gee about 2cm apart, in the steamer. Place the steamer over the boiling water, cover with the steamer lid and cook, in batches, for 4 minutes or until pastry is tender. Serve hot with extra hoisin for dipping.