Start by heating a little of the vegetable stock in a large pot. Sauté the diced onion, carrots, & celery. Add the potatoes. Add additional stock if starting to stick. Pour the rest of the stock in and simmer. Meanwhile start to make the noodles. In a large bowl mix the flour and salt. Cut the butter into the flour. In a small bowl mix the egg and milk. Add the wet to the dry slowly and mix until combined . Roll dough out on floured surface till it's about 1/4 inch thick and use a pizza cutter to make squares. Add add the noodles one at a time to prevent sticking to the boiling soup. Boil about 10 minutes. The soup should thicken a bit after adding the noodles. Add flour if you want a thicker soup.