Prepare the medium bowl, then add the flour, cornstarch, and salt. Gradually add water to it while mixing with a spatula in between. (i.e. Add one third of the water first and mix well with the flour. Add another third of the water and mix.) Do this until a rough dough forms.Be aware that you may need to adjust the amount of water you use depending on your room temperature. If it feels powdery, you need more water and if it starts sticking to your hands too much, you will need more flour. When you have to make an adjustment, do so in small increments.
Knead the dough for about 5 minutes or until it sticks together firmly, is smooth, and elastic. Put it in a plastic bag and seal well. Chill the dough in the fridge for 2 hours.
Prepare a medium-size pot and start boiling the soup stock over medium-high heat. Add the potato and onion. Boil for about 10 minutes.
While the soup is boiling, place a small bowl of water next to the stove. This will be used for wetting your fingers (mostly thumb and index finger) to easily stretch and pull the dough. It also helps to make the dough flakes thin, which means not a doughy bite!Take the dough out of the fridge. Hold the dough in one hand and use your thumb and index finger from your other hand to flatten it a bit. Pull it to stretch and thin the dough.Tear the dough into flat, bite-size pieces (2.5 to 4 cm or 1 to 1.5 inch size), then drop it into the soup as you progress.Repeat this until the dough is used up. This step can be quite time-consuming and requires lots of patience. It took me about 8 minutes to complete this step.
Reduce the heat to medium and add the soy sauce, salt, minced garlic, carrots, zucchini and mushroom. Stirring occasionally, cook for 5 minutes or until the vegetables are softened, but not too tender. Cooked dough pieces should look translucent and floating in the soup. Garnish with the green onions.