Heat up the vegetable oil in a wok.
Chop the spring onions, garlic clove and ginger, and stir fry for 1 minute.
Add the mushrooms, carrot and turnip, and stir for about 2-3 minutes.
Mix in the soy sauce, hoisin sauce, sesame oil and fish sauce.
Remove from the pan and allow the filling to cool.
Transfer the dough to the work surface, roll it so it can have about ½ cm thickness, then cut it in rounds of approximately 4-5 cm in diameter. You can use a larger cup to cup the circles.
Place a tablespoon of the mushroom filling in the middle of each circle, then seal well with your fingers, and shape into small balls.
Preheat the oven to 200 degrees C. (390 degrees Fahrenheit).
Arrange the balls on a baking tray, brush the buns with the egg wash mixture and bake for 10-15 minutes until golden.