How to Make It
In a large saucepan of boiling salted water, cook the fettuccine until al dente, about 12 minutes. Drain and rinse under hot water. Transfer the noodles to 4 bowls and keep warm.
Meanwhile, in a medium bowl, combine the soy sauce with the sake, water and sugar and stir until the sugar is dissolved.
Heat 1 tablespoon of the oil in a large skillet until shimmering. Season the beef with salt, add to the skillet and cook over high heat, stirring occasionally, until half-cooked, 2 minutes. Transfer to a plate.
Add the remaining 1 tablespoon of oil to the skillet. Add the onion and cook over moderate heat until lightly browned, 5 minutes. Add the shiitake caps and cook until softened, 5 minutes longer. Add the scallions and watercress and cook, stirring gently, until the watercress is just wilted, 1 minute. Return the meat and any accumulated juices to the skillet, along with the tofu. Add half of the soy-sake sauce. Bring to a boil and simmer just until the meat is cooked through, 1 minute. Spoon the sukiyaki over the noodles and drizzle with the remaining soy-sake sauce. Serve.