Place the cleaned chicken in a stockpot large enough to hold the chicken and 12 cups of water. Add the onion, garlic, ginger, scallions, and 12 cups of water. Bring it to a boil over medium-high heat. Skim off any foam on top. Reduce the heat to medium-low and simmer, covered, for about 40 minutes. Adjust cooking time for cut pieces of chicken. Start making your noodles (below).
Sprinkle the julienned zucchini with salt for about 15 minutes and then squeeze out the water. Briefly sauté in a heated pan with a little bit of oil. Set aside.
When the chicken is cool enough to handle, remove the meat from the bones. Shred the meat into small bite-sized pieces. Mix with the chicken flavorings: the mashed garlic, sesame oil, salt, and pepper. Combine well.
Strain the broth and remove the excess fat. You can line the strainer with a paper towel or use a fat separator.
Return the broth back to the pot. Add the onion slices, sesame oil, fish sauce, and soy sauce. Bring it to a boil and boil for a few minutes. Add salt and pepper to taste.
Make the noodles: Combine flour, starch, 1 tsp. kosher salt, 2 Tbsp. vegetable oil, and 1¼ cup water in a large bowl.
Mix with a wooden spoon to form a lump. Knead it by hand until it forms a ball. Put it into a plastic bag, seal it, and let it sit on the kitchen counter for 10 minutes.
Take the dough out of the plastic bag, knead it for 2 to 5 minutes, and then put it back in. Take out the dough and knead it again for a couple of minutes. It will be smooth.
Divide the dough into 2 balls. Dust a large cutting board with flour and roll the dough out with a rolling pin to a circle about 18 inches wide and 1/16 inch thick. Flip it over occasionally to make it even, round, and flat. If you need to, sprinkle some flour on it when you flip, to keep it from sticking.
Evenly spread some flour on the dough, and fold it over 3 or 4 times. Cut it into thin noodles, about 1/8 inch thick. Sprinkle some flour on them, and gently mix them up to separate them and spread the flour evenly.
Make noodles with the other ball of dough, repeating the steps above.
Bring the soup to a boil. You can add the noodles directly to this broth and boil until done, about 4 minutes. Or boil in a separate pot of salted boiling water then strain and add to broth — it all depends on if you like your broth a little cloudy and thick.
In each serving bowl, add broth, noodles, some chicken, and zucchini. Serve with chopped green scallions and Korean chili flakes on the side.
