Korean Hot Beef Soup

4 ounces flank steak, trimmed of all fat
2 garlic cloves, minced
2 teaspoons soy sauce
1 teaspoon ginger
1 teaspoon dry mustard
1 teaspoon sesame oil
3 cups beef stock
2 cups shredded Chinese cabbage
10 scallions, thinly sliced
8 mushrooms, quartered
2 teaspoons vinegar
8 ounces tofu, cubed

Freeze the beef until firm enough to slice easily, about 30 minutes. Cut across the grain into paper-thin slices and then into 1-inch lengths.

In a pie plate, combine the garlic, soy sauce, ginger, mustard and sesame oil. Add the beef and toss to coat. Set aside.

In a 3-quart saucepan, combine the stock, cabbage, scallions, mushrooms, and vinegar. Simmer for 10 minutes.

In a large nonstick frying pan over medium-high heat, sear the beef until no longer pink, about 5 minutes. Add to the soup, along with any pan juices. Add the tofu. Cook for 5 minutes to heat through.