Beef Sukiyaki Noodles

Name: Beef Sukiyaki Noodles
Ingredients: 1/2 pound dried fettuccine; 1/4 cup plus 2 tablespoons soy sauce; 1/4 cup plus 2 tablespoons sake; 1/4 cup plus 2 tablespoons water; 2 tablespoons sugar; 2 tablespoons vegetable oil; 3/4 pound beef sirloin, thinly sliced across the grain and cut into 2-inch lengths; Salt; 1 medium sweet onion, cut into 1/2-inch wedges; 8 large shiitake mushrooms, caps only, quartered; 3 scallions, cut into matchsticks; 1 bunch of watercress, thick stems discarded; 1/2 pound (about 1/2 block) firm tofu, cut into 1-inch cubes