Place the mushroom in a small heatproof bowl. Cover with boiling water and set aside for 10 mins or until tender. Drain. Use a sharp knife to remove the stems.
Meanwhile, place the noodles in a large heatproof bowl. Cover with boiling water and set aside for 3 mins or until tender. Drain.
Combine the stock, 3 cups (750ml) water, ginger, garlic, sesame oil and soy sauce in a large saucepan. Bring to the boil. Reduce heat to low. Add the gyoza. Cook for 7 mins until gyoza are heated through.
Divide the noodles among serving bowls. Top with pak choy and mushroom. Ladle over the hot stock mixture and gyoza. Top with egg and spring onion and serve with chilli sauce, if desired