For the noodles: Mix together the flour and salt in a stand mixer or by hand in a large basin. Slowly add the water with the mixer on low speed, and then knead or mix until the dough is smooth, flat and consistent. Cut the dough into small pieces, about 4 inches long, 2 inches wide and 3/8-inch thick. Cover them with vegetable oil, and place them in the refrigerator right away to prevent sticking.
For the soup: Mix together the sliced lamb, wine and egg white and marinate for 20 minutes. Coat a large saute pan with vegetable oil and heat over high heat until it reaches a smoking point. Add the lamb and egg mixture into the pan and cook until the meat is almost done, then set the meat aside in a small bowl. Add a little more oil to the pan and cook the scallions and onions without browning. Add the meat, cumin, soy sauce, sugar and salt and saute to let the flavor set in, 3 minutes, and then remove from the heat.
Bring a large pot of water to a boil. When the water is almost boiling, take 3 pieces of the dough from the refrigerator. Press the dough flat on a work surface and take one end of the piece in each hand and pull. Slap the dough on the table until it becomes elongated into a wide noodle. Rip this noodle down the middle, creating a longer, continuous, circular noodle and throw it into the boiling water. Repeat with the rest of the pieces. Boil the noodles until cooked through, and then drain and transfer to a bowl. Top with the lamb, and add more soy sauce, chili oil and black vinegar to taste.
Recipe courtesy of Jason Wang