Begin by slicing the flank steak against the grain into thin strips and marinate with soy sauce, sesame oil, black pepper, and red pepper flakes for 10 minutes.

In a large skillet or wok, heat olive oil over medium-high heat for 1-2 minutes.

Add the marinated steak in a single layer and sear for 2-3 minutes on each side until browned. Remove from skillet.

In the same skillet, stir-fry sliced onions and bell peppers for 3-4 minutes until softened, then add minced garlic and ginger for another minute.

Return the seared steak to the skillet, pour in apple cider vinegar and water, scraping the pan to deglaze. Cook for 2-3 minutes until heated through.

Garnish with chopped green onions and serve hot, either over rice or a salad.