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
Sesame seeds
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.