In a medium bowl, whisk together the egg, grated ginger, peanut oil, and salt until well combined. Add the flank steak and marinate for 15 to 20 minutes.

Mix the sauce ingredients in a separate bowl: broth, Shaoxing wine, soy sauce, Chinkiang vinegar, sugar, cumin powder, and red chili flakes. Stir until sugar dissolves.

Sprinkle cornstarch over the marinated beef and toss gently to coat evenly.

Heat 2 tablespoons of peanut oil in a skillet over medium-high heat until shimmering. Add the beef in a single layer and cook without stirring for 2 to 3 minutes.

Flip the beef to brown the other side for another 1 to 2 minutes. Remove and keep warm.

In the same skillet, add more oil if needed, then add green onions, garlic, and ginger. Sauté for about 30 seconds.

Add chopped bell pepper and baby bok choy, stir-frying for 1 to 2 minutes.

Pour the sauce mixture over the vegetables, stirring for about 1 to 2 minutes until the sauce thickens.

Return the beef to the skillet, tossing gently to combine with the vegetables and sauce.

Transfer to a serving platter, optional garnish with sesame seeds, and serve hot over steamed rice or noodles.