As the cooler weather swings around, there cannot be too much chicken soup. Hot-and-sour soup, both the thick Chinese kind and the fiery, clear Thai version, Mexican tortilla soup and casuela, Greek egg-and-lemon soup--they're all nourishing and so good. When it comes to heft, flavor and sheer eating fun, though, we vote for the nabeyaki udon at Sachiko. It comes smoking hot in a small iron pot with a wooden cover, which the waitress takes off for you, revealing noodles in a delicious, mysterious broth studded with treasures--green onion, tempura shrimp, a slice of red-dyed hard-boiled egg, scrumptious knobs of dark chicken meat and a just-poached egg ready to release its velvety yolk into the fascinating depths. Sprinkle with black sesame seeds laced with incendiary chile powder, and watch the dead come back to life. It'll cure what ails you.