1/3 cup soy sauce, more to taste
1/3 cup ketchup or 3 tablespoons tomato paste and a pinch of sugar
1 tablespoon rice wine vinegar, more to taste
A few drops dark sesame oil (optional)
A squirt of sriracha or other sauce, or a dried red chili to taste (optional)
1 pound egg noodles, preferably fresh.
Bring a large pot of water to a boil and salt it. In a smaller pot, bring 6 cups of water to a boil; once boiling, reduce heat so water bubbles gently.
To the smaller pot add soy sauce, ketchup, vinegar, sesame oil if using and sriracha or chili, along with a pinch of salt. Stir and let simmer.
Add egg noodles to large pot; fresh noodles will be ready in just a couple of minutes; dried will take longer. When tender but not mushy, drain. Taste broth and add more soy, salt, vinegar or heat as you like. Divide noodles into bowls and pour hot broth over all.
from Mark Bittman:
The noodles I use are fresh egg pasta, but just about any kind of noodle can be used.
Once you’ve made this once, you’ll probably want to take it to a showier place. Cook thinly sliced shallot, ginger or garlic in a little peanut oil before adding the water for the ?broth,? or add sliced celery, bean sprouts, snow peas or sliced carrots to it. Switch to rice noodles, soba, ordinary dried pasta or mung bean threads if you like. Crank up the heat a bit under the broth and warm or cook sliced (or leftover) chicken, pork, beef, Chinese sausage, shrimp, tofu or egg in it. Garnishes are obvious: scallions, cilantro, toasted sesame seeds or chopped peanuts come to mind.
There’s another option here, not quite as fast but with one less pot: cook the noodles, then remove them, and use their water to make the broth. It will be cloudier but a bit more flavorful.