8 ounces fresh or thawed frozen tteok (see note)
4 ounces beefsteak, such as chuck or sirloin, very thinly sliced
1/2 teaspoon soy sauce
2 teaspoons sesame oil
2 cloves garlic, minced
1 small onion, thinly sliced
2 cups green cabbage, cut crosswise into large pieces (optional)
1 to 2 tablespoons gochujang (Korean chile paste)
1 to 2 teaspoons sugar
2 scallions, cut into 1-inch batons
Soak tteok in cold water to cover while preparing the other ingredients, about 10 minutes. Drain on paper towels.
Combine beef with soy sauce, 1 teaspoon sesame oil and garlic.
Heat a wok or skillet over high heat until very hot. Add beef mixture and stir-fry just until lightly browned, 1 minute. Add onion, scallions, and cabbage, if using, and stir-fry until crisp-tender, 2 to 3 minutes.
Add gochujang and mix. Add about 1/3 cup water, remaining teaspoon sesame oil, sugar and tteok. Mix and let simmer until sauce is thick and tteok is soft, adding water a little at a time as needed. Adjust seasonings with sugar and gochujang.
Mix in scallions and serve hot, sprinkled with sesame seeds.
Tteok (Korean rice cakes, also spelled dduk or toppoki) are available in Asian markets. For this dish, the traditional shape is long cylinders; cut them crosswise in half before cooking. If using dried tteok, cook according to package directions and do not soak.