Fake Sukiyaki

In a wok over high heat, combine 1/4 cup of the beef stock, soy sauce, and sake. Add onions and mushrooms; fry until onions start to soften. Add the strips of beef and remaining beef stock, and cook, stirring constantly, for 2 minutes. Add spinach, cover, and cook for 3 minutes, until spinach is wilted, but not fully cooked. Adjust seasonings to taste, and serve over udon or ramen noodles.