Rehydrate the shiitake in a mug of boiling water for minimum 20 minutes. Once rehydrated, dice the shiitakes very finely.
Heat a medium frying pan with a little vegetable oil. Add the diced shiitake, spring onion and chopped pickled mustard greens.
Fry on medium heat for a few minutes before adding the soy sauce and stirring well. Once most of the liquid has evaporated, remove the pan from the heat and set aside an allow to cool for about 15 minutes before filling the dumplings.
Fill each dumpling with around a tablespoon of the filling mixture. I followed this video when folding my pastry but feel free to use your own method.
Turn on a steamer to high or set a bamboo steamer over a pot of boiling water. Line the layers of the steamer with perforated greaseproof paper and fill each layer with dumplings. Steam for around 5-8 minutes.
Serve alongside the bowl of dipping sauce and go crazy!