Using a cheesecloth, squeeze any excess water from 100 g white cabbage and 2 medium carrots (the other vegetables should be fine). Discard the excess water — this helps to prevent the vegetable gyoza from becoming soggy.
Add the cabbage, carrots, ½ medium white onion, 60 g mixed mushrooms, 3 cloves garlic, 1 inch ginger, and salt and pepper (to taste) to a large bowl. Stir well, then add 1 teaspoon soy sauce and 1 teaspoon cornflourThe mixture shouldn't appear wet; if it does, add small amounts of cornflour.
Take 24 gyoza wrappers. Spoon a small amount of filling into the middle of one gyoza wrapper. Spread water around the edge, then fold it in half, and carefully pleat the edges. Repeat with all the gyoza. While you're working, cover the gyoza wrappers and the complete gyozas to prevent them from drying out and becoming hard to work with.
Fry the potstickers**. In a large, deep-sided, non-stick frying pan, heat 1 tablespoons oil over a medium flame. Arrange the vegan potstickers on the bottom of the pan, in a spiral shape. Cook until the gyozas are brown on the bottom, or around 4 minutes.
Once the gyoza are nicely browned, quickly add around 60 ml water to the pan and cover immediately. Cook for around 4 minutes***, or until the gyoza wrappers are see-through and soft to the touch. Remove the lid, drizzle 1 teaspoon sesame oil, and cook for an additional minute or two.
Serve the vegetable potstickers with a traditional dipping sauce or incorporate them into a main meal. Garnish with spring onions.
Notes
* As noted in the recipe, you may need a little additional cornstarch to soak up the moisture from the soy sauce. Cornstarch is also called cornflour in the U.K. (Not to be confused with American corn flour.) ** If you don't want to make yaki gyoza (potstickers), you can steam, boil, or deep-fry your vegetable gyoza. *** If you're cooking the gyozas from frozen, you may need to steam them for up to 6 minutes. They take slightly longer than freshly made vegetable potstickers.