Make the filling. Put all of the ingredients up to the tofu into a hot wok or frying pan and sizzle for a couple of minutes.
Strain the tofu of all water and dice it. If the tofu is soft, place it on your hand and cut it on your hand then tip it into the wok. Fry the tofu for five minutes.
Add the spring onions and mushrooms. The mushrooms will release a lot of liquid. Fry until the mushrooms seem cooked, for about 3-5 minutes.
Prepare a jug or bowl with a sieve or chinoise. Put all the mixture into the sieve and let the excess liquid drain out. You don't want a very liquid filling as the gyoza skins will not hold it.
Make a little gyoza filling station on a table. Kids and friends can join in with this and more hands make quick work. Get a small bowl of egg white or a bowl of water (if you are vegan) next to each person. Take a stack of gyoza skins for each person. Put the large bowl of cooled filling in the middle and give each person a teaspoon.
Lay out the gyoza skin and dip your finger into the egg white or water, run it around the border of the gyoza/pot sticker/chinese dumpling/wonton skin. Put a heaped teaspoon of filling into the middle then seal the gyoza according to the pictures below. You want four folds on one side, which creates a crescent shaped dumpling.
Store them on a tray sprinkled generously with white flour, rice or wheat, so that they won't stick to the bottom when you remove them to steam.
Boil a kettle. On a medium to low heat, place a non-stick or well seasoned cast iron frying pan with a lid and add a little vegetable or groundnut oil. When the oil is hot, place the gyoza carefully into the pan, making sure they don't touch, and fry the bottoms until golden, 3 to 5 minutes, then add a couple of centimetres (an inch) of boiling water from the kettle. Stand back as this may splatter a little. Put the lid on. Then steam for 5 minutes or so. Using a fish slice, remove the gyoza and add a few more drops of oil, start the next batch. continue until you have finished all the filled gyoza.
Serve them hot with the reserved green part of the spring onion and the plum dipping sauce.