1 lb flank steak, thinly sliced against the grain

2 tbsp soy sauce (for marinade)

1 tbsp cornstarch (for marinade)

1 tbsp vegetable oil (for marinade)

2 tbsp vegetable oil, divided (for stir‑fry)

1 large onion, thinly sliced

1 red bell pepper, thinly sliced

1 green bell pepper, thinly sliced

3 cloves garlic, minced

1 tbsp fresh ginger, minced

1/3 cup soy sauce (for sauce)

2 tbsp oyster sauce

1 tbsp rice vinegar

1 tbsp cornstarch (for sauce)

1 tsp sesame oil

1 tsp brown sugar

1/2 tsp black pepper

1/4 cup beef broth

2 green onions, chopped (for garnish)

1 tbsp sesame seeds (for garnish)